The link Slappy provided is a good jumping off point, please take a look.
I'd use a siphon hose (like the one attached to gravel vacs) and sort of gently lift/swish the top of the sand while sucking up the gunk that gets up into the water column. What's going on with the lower part of your back glass? Try using a razor blade to remove it, could be more cyano or just a heckuva lot of green spot algae...
Step up your water changes, I'd say 25% once or twice a week. Personally, I'd load a ton more plants into your tank - fast growing nitrate suckers like water sprite, anacharis or just a mass of java moss. The best way to combat algae is to have plants that out compete them for nutrients. (This is after you've manually removed what's already growing)