Flatworms

[muck]

Staring at the tank today I seen a critter crawling on the glass. Im guessing it to be a 1/4 inch long flatworm. Ive read a number of posts about people using flatworm exit to get rid of them. A couple questions...

1. Are all flatworms bad though??
2. Should I be concerned??
3. Does anyone know a good site for more info??