Hitchhiker ID Please

[PowChickaPowPow]

Hi can someone tell me what this white thing is under my acan? The acan shrinked when I scraped the glass nearby and then I noticed this for the first time. Should I remove or let it be? Thanks

 

[Crabs McJones]

Pineapple Sponges. Harmless filter feeders

 

[Mark Goode]

Yeah, pineapple sponges. I used to have hundreds of them, but they've all gone now, both in sump and DT.