ID Please

[MyHouseIsAZoo]

So this appeared in my tank on Sunday. Tank is a 6 month old 13.5 gallon biocube. No one had noticed it earlier in the week, though we have been busy, so it may have been there for a couple of days. What is it? Good hitchhiker or bad hitchhiker? Only recent addition to the tank is a small piece of live rock from LFS to help seed coraline.

Eta: Nitrites - 0, Nitrates - 10, Ammonia - 0, PH - 8.4, Salinity 1.025
Water change was done a few hours after noticing hitchhiker and testing water. Hitchhiker was still there this morning.

 

[mcgyvr]

Just harmless sponge growth..

 

[Sugar Magnolia]

Yep, nothing to worry about.

 

[MyHouseIsAZoo]

Just harmless sponge growth..

Yep, nothing to worry about.

Good!!! I was worried that I had messed up and introduced a pest into the tank.