Mine do that all the time. They're opportunist lil' buggers!! I don't think you need to worry about it as long as the RBTA is getting enough food. Might help to feed the shrimp in another location of the tank prior to feeding the RBTA. I also don't think the RBTA will hurt the shrimp with their hard outer layer of skin. Reminds me, I need to feed both my RBTAs...lol.