He may well be a half-moon. I shouldn't try to say without seeing a picture of him fully flared.
Having a bubble filter, called a "sponge filter"will not only help keep his water clean, but will help you cycle his tank when it comes time to do that. No rush; wait for the bigger tank.
The silvery (oily?) film on the surface is a protein film which sometimes grows when the surface is very still. Over-feeding and/or under-cleaning makes it worse. A sponge filter or an airstone will usually disturb the surface enough to make that go away.