Angel with lump on upper mouth

My freshwater angle has had a growth on its upper lip for about a year.
(Yes, shame on me for not dealing with it sooner). The growth seems to
be getting bigger lately and in the last month the fish hasn't been eating
much. It's a mostly black angel, and the growth is greyish black and
currently about the size of a pencil eraser. Ideas on what this might be
and how to treat it? Fungus maybe? It doesn't look fuzzy...