From what I've read on other forums and care sheets red ear sliders can't eat out of water. Here's a PetCo care sheet that says that. Also, this biogeography from SFSU that says they only feed underwater. Have you ever seen your turtle eat out of water?
Yeah, they can't eat outside of water. I read it somewhere on this site that RES don't produce saliva, so even if they have food in their mouth they'll try to bring it underwater.