Here is the passage (Greek, English) quoted from Plato’s Republic (Emphasis J.W.) referring to "beyond being":
καὶ τοῖς γιγνωσκομένοις τοίνυν μὴ μόνον τὸ γιγνώσκεσθαι φάναι ὑπὸ τοῦ ἀγαθοῦ παρεῖναι, ἀλλὰ καὶ τὸ εἶναί τε καὶ τὴν οὐσίαν ὑπ᾽ ἐκείνου αὐτοῖς προσεῖναι, οὐκ οὐσίας ὄντος τοῦ ἀγαθοῦ, ἀλλ᾽ ἔτι ἐπέκεινα τῆς οὐσίας πρεσβείᾳ καὶ δυνάμει ὑπερέχοντος.
In like manner, then, you are to say that the objects of knowledge not only receive from the presence of the good their being known, but their very existence and essence is derived to them from it, though the good itself is not essence but still transcends essence in dignity and surpassing power. (Republic, book VI, 509b)
The passage is taken from Plato’s simile of the sun. Plato compares the idea of the Good with the sun:
The sun by its light provides existence and growth to the living beings and the sun light makes them visible for our eyes. Similarly also in the domain of ideas the idea of the Good creates by its force the ideas as stable and enduring entities and lets us recognize the ideas by our mind.
Apparently Plato does not express his main concept “The idea of the Good” by a clear concept. Instead Plato takes refuge to a simile.
It is no wonder that Plotinus, about six centuries later than Plato, does link his main concept “The One” to Plato’s concept “The idea of the Good” not on the basis of clear concepts. For Plotin’s interpretation and link to Plato I recommend Plotinus, Chapter 2.