pry up under it, score the attachment point via accuracy/surgery and remove it via abrasion in one fell swoop. they are easy to eradicate with a knife tip. you take some anchor point in the scrape, its a minor cosmetic damage until coralline plates back over. I rid my tank of them this way./
Just be careful when taking it off. maybe u can just frag the rock it's on? Which in the long run will be easier to re-attach. If not you would need some low flow and maybe a rubble containter and wait for it to attach.