As with any animal, you
can have too much of a good thing, if they don't have enough to eat and/or they eat the wrong kinds of things. You don't actually describe the snails -- are they stomatellid snails, or what? Do they look like the snails in this article?
WetWebMedia • View topic - Critter of the Week: Stomatella Snails
I'm going to go out on a limb here and assume that they are stomatellids, because that's the kind of snail that I've had suddenly appear in large numbers, and they are nocturnal. They might not be reproducing so much as just coming out of some live rock (especially if they are all a centimeter across!). I've had stomatellid snails show up in large numbers after the introduction of new live rock... Eventually, they run out of food, or predators reduce their numbers, to the point that the wave subsides.
If there is enough food to support your snails, you don't have a problem -- if they eat something like diatoms or nuisance algae, all the better. (Stomatellid snails eat diatoms.) But if they chow down on your expensive sessile inverts, that would be bad.
I suspect you are right about them being eaten by crabs. Do you see empty shells?
If you value them, move some to your other aquariums. If they are stomatellids, they may run out of food and/or be lunch for crabs to the point that they eventually get wiped out, but those snails would be a valuable commodity to have in your tanks (taking the place of turbo grazers or whatever), so it would be to your benefit to save what you can, maybe even give some away or sell the excess...
