Yeah that's fine. How big is it? I've never had a young slider show any interest in green food, big ones will take it though. Most market seafood type stuff should be OK for him. I would stay away from oily fish like smelt, bluefish, that sort of stuff...it's not great for them and it will really get your tank dirty. Shrimp, mussels, cut fish like tilapia, catfish, flounder etc are all good to give him some variety. Also earthworms/nightcrawlers depending on how big he is, and feeder insects like crickets are good.