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It's very hard to see but it believe it is a nudibranch. You may be able to ID it over that the seaslugforum.net. About a third of the way down the page starts the order Nudibrancia. A good bit farther down the page is the suborder Aeolidina. I'd start there. There's alot of critters to go through.
I have a few that came on some polyps as well, I syphoned one off but the other is small. Maybe my new sixline wrasse will eat it or ill wait till it gets a little bigger, they don't seem to do much damage when small