Either jellyfish or hydroid medusa - I'm leaning toward jellyfish in this case because the tentacles seem really short for most hydroid medusae I know, but it could be a hydroid.Are these jellys
Either way, unless it's photosynthetic or you have a bunch or tiny foods (like copepods, brine shrimp nauplii, coral food/small fish food, etc.) it should die off on its own soon.
