So, Hegel's objective idealism believes in manifestations of reason which is nothing but ideas. And which exists in this world is nothing but the ideas, everything is idea and idea is everything.
Objective idealism interprets the spiritual as a reality existing outside and independent of human consciousness.
So why is it considered different from realism ?
It is quite possible that connections between Hegel's Absolute Idealism and (some sense of) realism can be drawn out. But there is a fundamental divide between the two in the or a standard sense of 'realism'. It develops as follows.
Realism in most forms assumes the existence of a mind-independent world of which we can have knowledge. So for realism, mind and world are distinct. For Hegel, by contrast, any divide occurs not between mind and world but within mind or consciousness itself.
The following comments by Preston Stovall on Tom Rockmore's book on Hegel is helpful in explaining this view:
Rockmore's central claim is that most analytic philosophers commit themselves to metaphysical realism, or the belief that our knowledge grasps hold of a mind-independent world, while Hegel is committed to a wholly historically bound and constructivist theory of knowledge. (Preston Stovall, 'Hegel's Realism: The Implicit Metaphysics of Self-Knowledge', The Review of Metaphysics, Vol. 61, No. 1 (Sep., 2007), pp. 81-117: 86.) ...
The central evidence for Rockmore's ... reading of Hegel is culled from the Introduction to the Phenomenology of Spirit, where Hegel discusses a criterion for knowledge (the problem of how we know that we know) and the extent of the coverage such knowledge can hope for ... . The point of departure for Rockmore's claim that Hegel rejects metaphysical realism seems to be Hegel's assertions in sections 84 and 85 that "consciousness provides its own criterion from within itself, so that the investigation becomes a comparison consciousness with itself; and that "Notion and object, the criterion and what is to be tested, are present in consciousness itself. .. [thus] we are also spared the trouble of comparing the two and really testing them, so that, since what consciousness examines is its own self, all that is left for us to do is simply to look on." This line of argument is interpreted by Rockmore precisely as the renunciation of knowing the real world as it is. He writes, "[S]ince the distinction between subject and object no longer falls between a subject and an independent object, but rather within consciousness . . . the relation between reality and appearance . . . has been resolved." ... "[I]n the introduction to the book [the Phenomenology], for the canonical opposition between subject and reality, mind and world, or consciousness and what lies outside it, he [Hegel] substitutes an opposition within consciousness."
(Preston Stovall, 'Hegel's Realism: The Implicit Metaphysics of Self-Knowledge', The Review of Metaphysics, Vol. 61, No. 1 (Sep., 2007), pp. 81-117: 87-8; Tom Rockmore, Hegel, Idealism, and Analytic Philosophy (New Haven: Yale University Books, 2005): 23, 219, 221 and 222.)
The references to Rockmore do not do justice to the full detail or the direction of his argument but they do serve to indicate the distance and divergence between realism and Hegelian Absolute Idealism.
According to WP, Hegel is a proponent of absolute idealism. To wit:
It is Hegel's account of how being is ultimately comprehensible as an all-inclusive whole (das Absolute). Hegel asserted that in order for the thinking subject (human reason or consciousness) to be able to know its object (the world) at all, there must be in some sense an identity of thought and being. Otherwise, the subject would never have access to the object and we would have no certainty about any of our knowledge of the world... the absolute ground of being is essentially a dynamic, historical process of necessity that unfolds by itself in the form of increasingly complex forms of being and of consciousness...
That is, the subject is and becomes by thinking through a process known as the dialectic.
Objective idealism does as you say posit that the mental or spiritual exists outside independently of consciousness, notably rejecting naturalism. Both of these are forms of idealism because:
In philosophy, idealism is the group of metaphysical philosophies which assert that reality, or reality as humans can know it, is fundamentally mental, mentally constructed, or otherwise immaterial. Epistemologically, idealism manifests as a skepticism about the possibility of knowing any mind-independent thing. In contrast to materialism, idealism asserts the primacy of consciousness as the origin and prerequisite of material phenomena.
Realism on the other hand is material, physical, natural, and presumes that the universe gives rise to consciousness, not vice versa.
This is a traditional clash between those who feel their thoughts are primary, and those who feel the objects of thoughts, particularly in the environment are primary, and is tackled directly when on studies the philosophy of mind. In the Anglo-American tradition of analytical philosophy according to WP in philosophy of the mind:
Most modern philosophers of mind adopt either a reductive physicalist or non-reductive physicalist position, maintaining in their different ways that the mind is not something separate from the body
This is in contrast to the Continental tradition of phenomenolgy which has given birth to post-modernist thinking.
