Yellow Thing

[sunny sam]

Hi all,

I posted this on nano reef as my tank is 10 gallons, but I thought I'd try here too. Sorry for the poor pics. It's all I could do with my phone.

Any idea what this yellowish-greenish thing is (circled in red)? It's about the size of a dime and it looks like a sponge to me, but it moves around the aquarium. The first pic was taken on 7/20. The second pic was taken about the same time of day on 7/21. Circled in blue was its previous position on 7/20. It seems to move at a speed of about 2 inches per day.

Thank you.

 

[KrisReef]

Don't know, but it is interesting. I would guess flatworm or nudibranch that is mimicing sponge?

 

[sunny sam]

I think you are correct. Because it's often circular or triangular in shape, I had never considered a sea slug. I hadn't noticed any rhinophores, but I haven't looked too closely either. Berthella plumula is a possibility, but I imagine there are many other species that it could be too. Thank you.