I use tiger nerite snails. They only reproduce in brackish water so no population boom and 3 keep my 42 gallon clean. Hope this helps, I had that issue before. Perhaps cutting lighting back may help.
Just my 2cents
Cheers
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It's caused by high organics in the water combined with excessive lighting, are you able to post your water parameters? Are you using any Co2, or carbon supplement live Flourish Excel? How long is your lighting on? Any source of fertilizers?
Looks like a case of black beard algae. You can use hydrogen peroxide to kill it. Here is an article on the subject: http://www.aquarticles.com/articles/plants/Podio_Algae_Hydrogen_Peroxide.html