Tiny white star fish looking things?

[LukeMoss]

I'd try google or any topics on here but not sure what to even ask, here it is I've noticed that these are multiplying... I've had my tank set up for 4 weeks now and these are becoming more and more anyone have a guess? Can't even get a decent picture they are around the size of a pen nib... Thanks in advance!

 

[Rutrag]

Sounds like asterina stars. They are generally harmless, but a few folks in my reef club have indicated certain species have attacked their corals. I haven't had that issue with mine however.

 

[LukeMoss]

I have one of those already in my tank and currently haven't had any problems but these don't look at defined as star fish just have a circle centre and looks like legs but haven't seen any moving

 

[LukeMoss]

This is all I could get from my SLR, sorry it can't be any better

 

[Sniperhank]

Hydroids.

 

[LukeMoss]

Any idea as to whether these are good or bad?