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.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps