I'm having a difficulty of the concept of the mind in dual aspect monism. Is the mind a substance in this philosophy of mind?
You need to be mindful of the meaning of 'substance' in philosophy. It's completely different to the everyday sense of the term, which means 'a particular kind of matter with uniform properties'. 'Substance', in philosophy, is derived from Aristotle's use of the Greek term 'ousia' which means something nearer to 'being' than to 'stuff'. It was translated into Latin as 'substantia', which is 'something that stands under or bears'. So a 'substance', in philosophy, is 'the bearer of attributes'. So, 'mind' is not a 'particular kind of stuff with uniform properties', but something of which the following can be said: ability to grasp rational principles (etc.)
In philosophy "substance" is like "atom", which is the ultimate independent real existence not depending on other entities, is the unit of knowledge. In this sense, in most forms of rationalism mind is one type of substance or the only type of substance. Classical rationalists such as Spinoza held the latter view which is more monistic, while Leibniz held the first. Even in strict Dualism such as Descartes's mind-body interactionism, mind is a real substance there. Of course, in most materialism and modern phenomenology, mind is not a substance per se, but an emergent phenomena originated from underlying cold hard materials.