Algae eating snails are definitely beneficial as part of a SW
"cleaner crew". I always recommend getting at least 2-3 different types of snails, as all will *specialize* on different types of algae.
Algae needs 2 things to survive - light & food. Of course, our tanks need light to sustain the photosynthetic creatures we keep. Food for algae comes in the form of nitrates, phosphates, and DOC
's. Good tank maintenance (water changes, etc.), can go a long way toward nutrient export. However, we need to also limit the food at it's source - make sure you are always using RO
water so that you are not introducing phosphates with your water changes/topoffs. Limiting your bioload and insuring that your filtration capacity meets the needs of your tank will also limit available nutrients.
Snails are good at nipping potential algae issues *in the bud*, but cannot be expected to fully control algae if good tank maintenance is not exercised. (Not saying you don't, just FYI