It has been reported several times from very respected and trusted people here on this forum that they have had otherwise extremely healthy Haddoni anemones (in their care for multiple years), perish overnight after feeding silversides. Additionally, you should not feed foods larger than pencil-eraser sized to your nems. It is hard on them to digest large food items.
It is theorized that since silversides are not meant for human consumption, the supply chain is not as strict with keeping food quality regulations. Thus, it is possible for multiple feeze/thaw cycles to occur between the manufacturer to your freezer, thereby introducing possiblities for bacterial contamination, possibly due to potentially unsanitary conditions at the plant (not food product = not food regulated = possible pathogens).
It is best to stick with raw, food-quality seafood found at your local grocer cut into small pieces.