Originally posted by JimLewis What you want in this situation is Copper Nitrate or Copper Sulfate. This kills moss as effectively as Ferrous Sulfate (iron) but without the resulting rust stains that you would get from iron.
Sealing the concrete helps with removal of future moss. It won't prevent moss build up. But it does make it much easier to wash off.
Trim the trees above it. The reason it grows is because it is a moist, very shady or dark area. If you have the option to get more light in the area, the sun will do the work for you. If that is not possible then you will have to do applications like Jim stated.
Yah, actually Round-up should work great too, as long as there's not anything else important around that the round-up would kill.
To answer your question above; No, you won't find Copper Nitrate or Copper Sulfate at Home Depot. LOL. You will have to get it at a store that specializes in fertilizer and herbicides. They are hard to find but these stores are all over. You may have to drive a little.