Sounds like you have young ones, that is why they are darker and still have
their top spirals. MTS's are notoriously hard to get rid of. Like other
posters said, they do perform a good function by turning over the substrate,
but you gotta keep them in check. In my experience, loaches will not do the
trick on MTS, and the snailicides don't work too well (especially if you
have fish sensitive to meds like corys) as the snails can clamp their trap
doors (operculums) shut and keep out the chemicals.
Best way to keep them out of you filter is to keep the numbers low, using
traps, and if you want to try, using some loaches.

-Seweryn
I still would like to know how they reproduce/multiply and if they are not
harmful to my tank then any ideas to at least keep them out of the filter.