Brittle star?

What is this? I had 5 of them come out of my live rock tonight when I moved everything. I have never seen them in the tank before.

They have 6 legs and were standing up and spewing out a cloudy substance.

Looks like you have the little brittle stars in your tank! They are friendly critters and if they are spewing, you will likely have more soon!

Mni brittles! They come and go in my tanks, sometimes they are everywhere, right now not seeing too many. Harmless but adorable.

Yeah what these Gals say.
looks like Ophiactis spp.

common in reef aquaria and they do breed

great "reef janitor"
If you have Tonga Nass Snails if they get hungry they will eat your mini starfish.

I was actually planning on getting some of those snails, they are the ones that burrow in the sand correct?

Yeah ,I watched my Tongs's come out of the sand and Attack and start eating them when I ordered 50 MBSF from a hobbyist. my Vibex Nass went after the parts the Tonga's left behind but showed no sign's of actually attacking and killing like the Tonga's did. Ilyanassa Nass I have never seen go after the stars but then again there more Veggie dead matter Scavengers were Tonga's and Vibex are more carnivorous.