Unknown

[Damaster]

Can anyone tell me what is this that is growing on the rocks. It begining to grow all over the rocks on the gyros in the overflow box.

 

[Damaster]

I was told it was

Invasive tunicates​

 

[ISpeakForTheSeas]

It does seem to be a colonial tunicate species - possibly Botryllus schlosseri (A.K.A. Golden Star Tunicates). Based on how it is spreading, it does seem like it may be invasive in your tank. Some people like to try and starve them out, but my guess is that manual removal will probably be your best bet if you want to try and control their spread.