at first glance maybe a vermetid snail but closer inspection makes me believe its maybe a small feather duster worm. if its a vermetid snail just try and clip it of with a small tong at the base without harming the coral and it should be gone unless the vermetid snails tube is deeper inside which you would hope when you clip it off the snail is at the top of the tube and not slithered down inside the tube.