Their are corals that do not require lite at all but I do suspect you can grow watermelon mushrooms in 2 watts as well as a wide variety of polyps. Non- photosynthetic corals - Red + yellow deep water gorgonians, Purple sea fans, blueberry gorgonians, purple rod gorgonians, porous sea rod, yellow or white sea rod, golden sea fans, and LPS sun coral both black or orange to name a few. Other critters - sea cucumbers, feather dusters, sponges, flame scallops, pizza and dwarf carpet anemones.
HOWEVER - you do have to feed all these guys 3 types of planktonic foods - Phyto, bacterio, and zoo. This will mean your fighting nitrates + phosphates.