It is Planaria. Normally growing in shrimp tank. This is due to too much waste or left over. If you cut it into 2 pieces. then 2 broken pieces will grow. If you cut it into 10 pieces..then you will hv 10 more extra.

I once made a trap to catch them. goto www.shrimpnow.com. and looking for my thread. Or else, put salt... but then u gonna be carefull with yr mosses. Salt kill planaria..but kill mosses as well.