What does the Roman Catholic church have to say about monism (the idea that there is no difference between God and his creation)? Meaning creation in some sense is literally God (with or without a central conscience, and with the exception that God is comprised of the Father, Son, and Holy Spirit), or its part of God's body or form (I am aware that God showed his form to Moses, but perhaps in a different way or maybe its part of the form of one of God's [please excuse the terminology] components [the Father, Son, or Holy Spirit]).
It just so happens that the Catholic Encyclopedia has an article on this. Their definition is much broader than the one you provided. It actually has extensive notes on several meanings of the term. the section in the term in the context of theology is as follows:
The term Monism is not much used in theology because of the confusion to which its use would lead. Polytheism, the doctrine that there are many gods, has for its opposite Monotheism, the doctrine that there is but one God. If the term Monism is employed in place of Monotheism, it may, of course, mean Theism, which is a monotheistic doctrine, or it may mean Pantheism, which is opposed to theism. In this sense of the term, as a synonym for Pantheism, Monism maintains that there is no real distinction between God and the universe. Either God is indwelling in the universe as a part of it, not distinct from it (pantheistic Immanentism), or the universe does not exist at all as a reality (Acosmism), but only as a manifestation or phenomenon of God. These views are vigorously combated by Theism, not only on considerations of logic and philosophy, but also on considerations of human life and conduct. For the ethical implications of pantheism are as detrimental to it as its shortcomings from the point of view of consistency and reasonableness. Theism does not deny that God is indwelling in the universe; but it does deny that He is comprised in the universe. Theism does not deny that the universe is a manifestation of God; but it does deny that the universe has no reality of its own. Theism is, therefore, dualistic: it holds that God is a reality distinct from the universe and independent of it, and that the universe is a reality distinct from God, though not independent of Him. From another point of view, theism is monistic; it maintains that there is but One Supreme Reality and that all other reality is derived from Him. Monism is not then an adequate equivalent of the term Theism.
The key sentence in there is this:
In this sense of the term, as a synonym for Pantheism, Monism maintains that there is no real distinction between God and the universe.
if you follow the link to their article on pantheism, you get this clear statement:
The Church has repeatedly condemned the errors of pantheism.
Therefore, the Catholic Church's official position is that monism is an erroneous view of God.