White fuzz growing inside GSP??

[JakesReef209]

So I’ve had this gsp for a couple of months now and it seems very happy and healthy when it’s open! But at night when it detracts you can see this white fuzz that seems to be taking over its mat. Anyone know what this is?? Any info is appreciated

 

[KJ]

Sponge.

 

[JakesReef209]

Sponge.

Oh ok, is this common? Are they harmful?