What are these, I found them thriving on live rock in my saltwater aquarium?

Yes, that looks like some type of soft coral or anemone. Either a zoanthid [sea mat] a href="http://www.zoanthids.com/" rel="nofollow"http://www.zoanthids.com//a or some other type of *on* polyps. Too bad they aren't expanded more to be able to tell exactly what they are. At least they don't appear to be Aiptasia. a href="http://www.reefcorner.com/SpecimenSheets/aipatasia.htm" rel="nofollow"http://www.reefcorner.com/SpecimenSheets…/a