In my experience no snails are great algae eaters, even though they may peck at it here and there.
Your best bet would be something like Oto's or shrimp, depending on the size of the tank. The brown blotches sound like diatoms, and it's easily wiped away by hand.
If you have scum on the gravel, you might want to consider vacuuming/cleaning the bottom a little more thoroughly, and feeding a little less.
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