Yepp, Hydroid medusoid. See page 3, post #51 of my microscopy thread. They will most likely disappear after some time on their own. They appear mostly in new tanks, but if you upgraded tanks or moved everything into a new system they can also appear in my experience.
You might also notice some tentacles soon (see page 4, post #61) or these guys (see page 4, post #80). Both can irritate corals which is annoying. The tentacles will probably also disappear on their own while colonial hydroids might require you to become active depending on species.