Kind of nasty looking, huh? Sorry--can't identify that one. Could be a nudi of some sort--they are more prone to having frills and extensions than common flat worms, while few nematodas have anything resembling that. In shape, it looks more like some of the parasitic copopods (they vary from the good type substantially), but also look more armoured than a typical worm. Wish I could be of more help! One plus--worms/pods that are found free swimming are seldom parasitic, but can be predatory. Watch to see if it spends more time on a sponge or invertebrate.