I dont think it has anything to do with it really, as the rock work was added a month ago and this was literally overnight. Ive used the marco rock in several other builds before without any real issue, and so I dont think its that. If anything I think it might have something to do with the clam going sideways. The derasa hasnt had much in the way of shell growth in a few months (since February really), and so its likely it has been dying slowly over the past few months and when it finally kicked the bucket, the alk demand dropped dramatically, and since my alk is generally pretty low (6.8-7.0 dKH) a spike to over 8 probably just tore up the acros. Even after getting it back down (7.0 dKH currently) things are still a bit up in the air at the moment.
The new rock is looking nice now, actually. Its gone from a bone white to a more natural green-gray and starting to grow coralline in spots. I think the only real wildcard there is the plastic coralfix, but that type of plastic is pretty inert typically.
I feel like I've read somewhere that a clam dying is a pretty giant nutrient spike. Maybe there was a quick ammonia bump?