Thought it was a pineapple sponge but…

[Abiodork]

It now appears to have sweeper tentacles. It’s in my refugium and has been stationary for many weeks. The main body is maybe approaching nearly a quarter of an inch. I’m about to “when in doubt take it out” now that it appears to have tentacles, but it’s pretty darn cool and if it’s not a nuisance I would leave it. I do have a hermit and a turbo snail that I’d like it to leave alone. Thanks for any thoughts or ID.

 

[ISpeakForTheSeas]

I'm struggling a bit with the pic personally, but look up Tethya gracilis and see if it's a match:

Looks like it may be Tethya gracilis - it should be a harmless filter feeder.

They're not super well known (they were discovered first in a German aquarium), but the study I've found describing them was very informative (linked below).

They grow up to ~1.5cm (though they can contract to be ~80% of their max body size); they can slowly move/"walk" - reportedly, they can move several centimeters a day (especially when smaller), and they do this to remove/shake off sediment that gets on them; and they reproduce by budding (so they'll drop pieces that grow into new sponges).

https://www.researchgate.net/publication/215665065_Three_New_Species_of_Tethya_Porifera_Demospongiae_from_German_Aquaria