I believe it is Porpita porpita aka "blue buttons".
It is an interesting beast similar to the portugese man o war - a colonial animal. Not a jellyfish or anemone. It sits at the ocean surface, and is not likely to survive in a tank I should think.
Another similar animal is Velella velella, the "by the wind sailor" which has an interesting "sail" on top.
These were classified with the portugese man o war Physalia as siphonophores but I see now they have been removed into the Chondrophorina.