the ones near the center of the colony are, at least I think it's fair to guess, the oldest so have had the longest exposure. Perhaps this is a response to try to shade themselves from the lighting, there's still a lot of conjecture out there as to why corals do this even that it's some form of coral cancer, and it's really odd that the shroom lost its bubbled up vesicles. Isn't it cool when you pick up what appears to be a dud then get it under some better lights and it turns into something awesome?!