Is your tank fairly new? It's common in new setups and will eventually stop showing up once your tank has cycled completely. The best thing to do is just rub it off (you can do this wish any sponge or alge remover) and just wait out the cycle. Adding more fish won't help the situation, really.
Just want to add: Oto cats are good for eating brown algae, but only should be put in mature tanks, as they are sensitive species. So, depending on how long your tank has been set up (and your fish selection), you can add otos.