Sponges in overflow?

[Shifter]

can anyone help ID what this is? Any suggestions to help keep them under control? I’m thinking of adding the skimmer, but other than these everything else is looking good. They started in the return compartment in my fluval 13.5 and have started to spread.

 

[martin.a]

This may look like Sycon sp. (a calcareous sponge AKA Pineapple sponge) covered in detritus. But it really is difficult to say from this photo. Maybe extract one or two, clean them up by blowing water on them, and post another picture?
If these really are Sycons you do not need to worry. They are fine. They usually come and go. I would love to be able to establish a stable culture.

 

[Timfish]

What Martin.a said

 

[ColorMeGone]

If it was my 13 gal. tank I would not use anything in the overflow and for sure no skimmer. If you have lots of rocks and good flow and your numbers are good then that is all you need for filtration. JMO