Help! ID of this mystery pest

Hello all,

I am new to this forum - however I will confess openly I have read threads and topics on here for years and have found the answer to every question that has arisen, so thank you all!

However, I have a small colony of zoas and I have a mystery pest on it. It looks a bit like a tick you get on a dog with small antenna like bits sticking out. It's black in colour. I will put a picture on to help identify it.

Any help on knowing if it's harmful, or not would be great.

Thanks again...