Pardon my asking, but why is it so important to remove algae from the back of the tank? Algae is beneficial in many ways. While I personally can't stand to have algae on the front and side of my tank, having it on the back generally does interfere with viewing and it gives your fish something to nibble on as well as generate some oxygen and host good bacteria.
If you have resolved to get rid of it, and you have a glass tank, then I sould recommend a regular razor blade on a stick of some type, these are commonly available from pet store and similar products from hardware stores. Otherwise some Otos would probably do the trick if you wanted creatures to do it. Chemicals are almost always a bad idea.