What's on my zoas?

Hello everyone! What's this on my green bays? I've had this frag about 2 months now and it's been back and forth in health. So I started giving short ro/di dips every couple days and followed up with a peroxide dip to clear off some algae trying to grow around it. I've seen this tan crusty stuff a few times and removed it. Now it's back again. What is this? It is kinda crusty but comes right off with tweezers.



Thanks!!!!