I had something like this happen once, my salinity went up and I started getting tons of algae on one of my live rocks. I think the bacteria in that one rock died and caused the algae to start feeding. I think the reason it happened to only one rock was because it came from a different system than my other rocks so the bacteria may have been a bit different. The problem went away after a few months when the rock recycled.

Ahhh sorry to hear that. It’s super annoying when your tank is maturing and thriving all to be hit by a plague of algae. Bounce back is all one can do lol. Open up the wallet and hold your breath as you slowly restock.