What are these things on the back of my tank wall?

[testuser]

Per the attached picture, what are these brownish clumps of things growing on the back of my tank? Are they harmless?

 

[ISpeakForTheSeas]

Pics aren't super clear, but I'd guess those are sponges. If so, they're most likely harmless filter feeders.

 

[Cell]

Can you get a more clear pic? Are they moving around? They almost look like nudibranch's.

 

[Cell]

Oh nm you said growing.

 

[testuser]

It's hard for me to get a clearer picture, I guess because of the perspective. Was hoping that one of them would have a clearer one. But yeah, they don't move when the water flow is turned off.