Mystery snails don't feed exlusively on algae. They are scavengers and will graze on things like biofilm, dead plants and animals, basically anything they can find (including algae). You can supplement their diet with blanched vegetables every once in a while. If you have such a massive algae problem there might be more to it than light alone. There is most likely a nutrient imbalance in the tank, caused by a buildup of organic waste from either overfeeding, poor maintenance or bad tap water (if that's what you're using for water changes). Could you give some details on your tank and the issues you're having? Do you have a filter? What's your water change & feeding schedule? Do you have any live plants? What are the water parametres? We can give some constructive advice but not without additional info.
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UV sterilizer might help, if you can get your hands on one. It's sompletely safe for all living organisms. But first I'd make sure tank maintenance is up to par, and there's no organic waste lying around. Lots of fast-growing live plants like hornwort or salvinia will also help combat the algae by competing with it for nutrients.
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