Hi lovable angel,
It's true that the only sure way of sexing angels is to observe them at spawning time. The female's ovipositor is much more visible, round and blunt. However, experienced angel owners soon become fairly able to spot a gravid female, even before she's in full breeding form. (gravid = full of eggs; her belly will become nice and round and full). You may hear other things about males having a rounder crown (forehead) and such, but those are not true ways of determining gender.
As for the brown crud, as was said, it's probably algae. Are your plants real or plastic? How much light does your tank get? Any idea what your phosphate levels are like?