[GeToChKn]

Yes, that's shrimp poop. Part of the reason most people use black or dark substrate, can't see the poop. It also helps the red shrimp look more red. Shrimp, try to a very small degree, to blend into their environment, so on a darker substrate, they will be a bit redder and darker as they try to blend in. On a lighter sand like that, they will go a bit lighter. Decent fire reds shouldn't go very much lighter though. Cherries can go almost clear though as they aren't as dark as the fire reds.