Well, from my expierence my anemones prefer tiger shrimp pieces (fresh), it's meat is easier to digest rather than whole fish. Other than that, there could be other causes:
1) Anemone is not than hungry
2) Silverslide is too big for an anemone
In any case, remember that anemone will spit it's food out, and it will never be completely digested - it's the way anemone's digestive tract works. However, it will not happen after several hours, 12-16 hours is more common.