Pineapple sponges, harmless filter feeders. You usually see an explosion of them in a new tank, or a tank that has excess nutrients.
When my tank was new, up until it was roughly 8 months old, I had a new growth of these every week even though I was scraping them out with my weekly WC's. Once my tank matured, they virtually disappeared(I still have some in my sump section, but no where near the population I used to have).