Simple changes in water flow and light intensity can change the color. Id the smooth brown frag on the right the one you are talking about? And is the colony you fragged from behind it? That will help with the ID. If you are looking for an ID on the larger Acro in the back i would say maybe Acropora selago. The one in the front right is too small to even guess. Do you have a pic of the mother colony?
Corals of the World Factsheet and images - Acropora selago
I agree it can take time for some Acros to bounce back. So don't keep trying new things to speed up the process. It could do more harm than good. Were you IDing it as Bonsai? Or did the OP have a different one you wanted? IMO if they are looking for an ID it can't be Bonsai. No lineage.