It is hard to tell what will happen, it really depends on how badly ur clowns want the anemone. I would figure that the crab will soon leave them alone and all will be okay, but that is only because of my experience. I had my clarkii hosting their anem a lil while before introducing the crab. Then the crab was a lil bit of a pain, but soon calmed down. This time it sounds as if your crab may have first dibs, so time will tell
You can get him out, but don't go digging around on your anemone, remember its delicate. Get some tongs and, if u have one, put him in ur sump for a while. Then you could try putting him back after the clowns are hosting.