Are you sure it is a real G.A.R.F. Bonsai Acropora? IMO there is only one true Bonsai, and that came from G.A.R.F. That coral gets falsely labeled more than any Acro that i can think of. There are many species of Acropora that require different care and can have reactions to changes in the tanks. If you have two bonsais one has to be a fake, if not both. There are dozens of different species of Acropora that can be purple with green polyps. Calling them all Bonsai Acropora seems irresponsible. So that can make this very difficult to determine what is what, and what to do to fix it.
This is exactly why it is harmful to misidentify a coral with an existing show name. Post some pictures and maybe the true species can be identifies or the problem can be resolved easier at least.