I have been in this same situation, first I would consider safety for the nem. Normally when a nem lets loose there is a high concern of the nem ending up in a power head that can damage it. Personally I would remove it from the snail and try to place it back on rock. If you do remove it from the snail don’t do it forcefully as you can damage the foot. Holding it in a higher flow area with direct flow on the nem should cause it to release in my experience. Always remember patience is the best thing you can add to a reef tank, nothing good happens fast.