Sea Monkey is a marketing gimmick, they are actually Brine Shrimp! Adult Brine Shrimp do not provide enough nutritional values thus most reefers feed Baby Brine Shrimp.
Mysis are Fresh Water Shrimps, they are reported to have higher nutritional values and plus due to the fact that they are Fresh Water, it is belive that they do not harbour parasites that will threaten Marine Fishes.