Jellyfish lookin things????

[aquakid1]

Hey all ,Brendan here , I got these tiny little brown jellyfish things that attach themselfs to rocks..... there are littlerally hundreds. After they have been on the rock for a while they start to pulsate (well there stem that attaches them to the rocks does) after this they detach themselfs and bob around.....( if u can imagine a jellyfish swim thats how they swim) they are only about 4mm wide ,they are brown , and they resemble a tiny umbrella, also they dont harm my fish ( 1 regal tang, 1 rabbit fish, 3 green chromis, 2 ocellaris clown fish, 2 neon gobies and a cleaner wrasse)
the tank has been running for 5 weeks now and i bought it all second hand with fish already in it..... this problem has only been going for about 3 weeks, they dont cause problems but they annoy the crap outa me how they swim very stupidly around!?!
from brendan
p.s- this is my very first post on this board ( im a new member )

 

[HK StYle]

i got no idea, but it will be helpful to provide a picture

 

[danmhippo]

It's called upsidedown jellyfish. Do a google search should yield some result. I had them once. Hitchhiked into my tank and breed like rabbits. Suddenly die-off for no apparent reasons.

They do not possess very potent stings and get along with most critters fine. They do need some currents to keep themselves bouyant though.