First, I've found that ay large, comparatively dense food like shrimp and silversides should be mashed up first, otherwise not only will the anemone have difficulty digesting it, when the food is expelled, it will be almost still intact and left in the tank, could cause a N02/N03 spike, depending on your system and clean-up crew.
Second, they'll eat pretty much anything. My RBTA has done very well with flake food and tank water in a turkey baster.
As long as the anemone's foot is attached to a substrate and it looks healthy, you have nothing to worry about.