You could use a turkey baster, but I would recommend trying to siphon it out when you do your next water change.
I know you said it came on the coral you bought but you should also try to figure out what nutrients it is feeding off since it has started to spread in the tank.
The black out might help but you new SPS coral is not going to like that.
Start with the siphon then if you need to try a black out or slime away.