Suddenly popped up...

[Daddy-o]

I am at my brother in laws and has this thing that up that suddenly pooped up this week.


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[melosu58]

Looks like Hydroids to me. They are not a good thing. They will sting your corals.

 

[Sniperhank]

Remove the rock from the tank. Burn those things with fire. They can be awful to deal with and will spread.