looks like a aiptasia i have some pics ill try to post soon for you and how to get rid of it
heres a link with some info aswell http://aiptasia.ca/
is there any thing in the tank that you want to live?
you will need to trake out everything not infected and place them into another tank
untill you have resolved it
ok fist do not reach in your tank to rip them out. they carry spoars and they will release them. the safest way that i can see is boil RO
water and kalk
till it looks pasty then suck it up with a turkey baster than inject it into it as soon as you inject it try to suck up what ever comes form this species