Welcome to TRT! I'm siding with Jerry's guess........look like a big batch of Pineapple Apple Sponges to me. Very common, and nothing to dread, and will come and go as the environment suites them.....WHAT THAT IS can't tell Ya though.....SORRY!
It looks like a Pineapple sponge to me too! Congrats, they are a great filter feeder, and I have them in all my tanks as well. It's actually kind of an indication that the environment is healthy and stable.