It's a tunicate or sponge like mentioned above. harmless forsure, it could die off on its own or if it doesn't than no big deal unless you don't like it
So AdamNC I think I have same thing..mine feel kinda hard, is that like yours...I read if u pop them they have spores inside I thought they'd be soft...have u tried removing &!how??? Thx
Heres mine growing at root of a coral.
I'm still plagued by it but it's not uncontrollable. Usually as I do water changes I'll scrape the hose end against it and pop it. If there are spores then they are immediately sucked out by the hose. On smaller pieces of removable rocks I just take the rock out and scrape the bubble algae into the trash and then a quick dip and swish in some fresh made salt water. And yes they are firm to the touch.