Diario de floundark

Okay, so around last year, I found a Polychaete (I think). It was dull out of water and vibrant in water. it had two giant brown spots on one end. it looked like it had poisonous markings, I picked it up, and never had any reaction, so I don't think it was poisonous. It was very odd. I am still confused as to what this is. it almost looks like a long anemone. it rarely moved. does anybody know what this is?

hello. Recently I went out to Sandy Creek Park, N.C to go net some aquatic life. I found nothing out of the ordinary, water bugs, fish, and snails. but I found an Eastern Mosquitofish (Gambusia holbrooki) with a streak marking on each eye. at first I assumed disease, but I looked at other specimens I've caught, and none have it. I then looked at pictures nearby I did NOT take, and they had similar markings. So then I thought if I could figure out if it was male or female, maybe it was sexual dimorphism. I figured out by body shape it was male, but only some males have it, and some females too. I then thought they may be a winter camouflage, but another male I caught on the same day too didn't have it. I then thought breeding season markings, but that was again disproven by another male caught on the same day not having it. So why do some have it but some don't? Could someone please tell me what this is?