I do not see any similarity between Angels and Moonlight Butterflys other than being in the air... Besides the M.L.B Boss, the rest are idle unless you bother them. Angels are basically sentries tethered to a Pilgrim tree, they are intended to scan the area and limit movement until you find and kill the tree. So different tactics, roles, damage, aggressiveness, mechanics and origins... But other than that, totally similar.