The white marks are typical?
I used to like feeding mine clams on a half shell or rosey minos left over from my brackish tank. Just remember to wear a glove when getting your hands close or use tweezers.
Ps I thowed and removed the shell before feeding mine clams.
Unnecessary, they rarely eat things and get most energy off of light. I've never fed mine and it's fine. If the food is too big it'll stress it out along with use more energy than it gets from it.