I had the clear ones too once. All they ever did was eat algae off the glass. Fearing they would become a nuisance I added a sixline wrasse. The population eventually crashed. These were not the coloured ones that you can easily see, they were very small and translucent. Basically the same ones you're talking about (Joel- unfortunately I think you have a different kind
)
Anyhow, mine only lasted a few months. Whether it was the sixline or not I can't say. I never saw him eat the flatworms so it's possible it's just coincidence.
Fast forward 4-5 years, I still have the sixline and he's now a bit of a holy terror (they're not as nice when they're adult as they are when they're juvenile). So be careful about sixlines.. don't have docile tankmates that have to share their food source (mandarins, dragonets, other wrasses, etc.).