Its prolly a mts, I'm pretty sure they come hitched on plants. I actually got a plant (java fern) mainly because of all the snails I saw on it. They will burrow under sand or hide under deco in gravel I guess. At night time they come out and eat. There fine, and help control algea a bit. They will reproduce, until the tank can't support them, but it won't hurt your fish. There good little cleaners. If you want them gone, get some assasin snails. Like, 3 or 4. They eat other snails at about a snail a day each, and they look pretty cool.
A good trick if u dislike these snails is to dip your plants in hydrogen peroxide, and its all good, because in light, it becomes water and o2 ( I believe that what happens) people on here also reccomend it for spot cleaning algea, but it kills all the germs, and I guess snails haha before it goes into your tank. IMO, id leave them be, they do more good than harm.