Ok, I looked it up :-p Mysis vs mysids according to someone on reefcentral:

Mysis Shrimp is a Mysid Shrimp, but Mysid Shrimp is quite a general term which incorporates many species of shrimp in both fresh and salt water. Most commonly when a marine aquarist refers to either growing Mysid Shrimp and usually when they find a Mysid Shrimp hitch hiker, they are often talking about Opossum Shrimp (Mysis relicta). But not always. More often than not, it's difficult to identify a shrimp beyond Mysid, so Mysid is what is used.