MTS (Malaysian Trumpet Snails) can live in full marine , they are extremely hardy snails and adjust quickly to the increased salt content of a full marine system.
Once they get established they are nearly impossible to get rid of without doing a full tank breakdown and cleaning it very very well with a bleach solution over a 2 week period, Anywhere that is damp, the snails will automatically go to and will start to breed there.
You have to clean EVERYTHING, filter(inside and out if you run one), live rock, literally everything in the tank that comes in contact with water. It is not an easy process and will almost certainly kill any live rock you have.
There are ways to get rid of them to an extent but again this is not a final solution. Try putting a piece of lettuce leave in the tank overnight and by morning it should be covered by snails, any CUC (clean up crew) in the tank will likely be on it as well, so this is best done without the CUC present.
The only other way is by using copper medication but again with marine this is a nightmare as it will kill all your invertebrates.