These are usually photo synthetic. They love to hang out on euphyllia polyps, a 5 minute dip should get rid of most if they're on your coral. In all likelihood they are, if there are that many on the glass. Typically not harmful unless they get in the hundreds and you kill them all at once with flatworm x or something.
I tried manual removal. Couldn't even put a dent in them. FWE twice, the 2nd time at 2x dosage. Knocked them back but still had way too many. Got a Blue Star Leopard wrasse pair. They eradicated them from my tank in about a week.
Beware the green coris wrasse. I had one for about a year that seemed like a model citizen. Then it went rogue and started killing it's tankmates. I tried nets, bottle trap, etc. I finally resorted to a #10 barbless fly fishing hook to catch the little bugger. I took him to my LFS where he was placed in the predator tank with large lions, triggers (including a undulated) and a couple eels. He held his own.