This Is A Rock Or Glass Anenomae, They Are Bad In A Reef, They Can Sting Your Coral Causing Problems For Them Or Any Small Creature Who Comes Across It. The Easy Way To Get Rid Of Them Is To Put A Couple Peppermint Shrimp In, Or If You Want To Add A Fish, A Copperbanded Butterfly Will Also Munch Them Up And It Is A Beautiful Fish. There Is Also Treatments To Inject Into Them To Kill Them, Do Not Cut Them Out Though!!!! They Will Spread Like Wild Fire!!!
Hope This Helps!!!
Good Luck