They aren't mysis like what you buy in the way of frozen food. Those are a freshwater variety.
However, there are small mysid type shrimp that most people have in their tanks - the picture confirms what I speculated earlier.
They're a good thing to have. They stay small like that - those are adult size, and they will reproduce. They are good to have in the tank and your fish can and will eat them. Your chromis was probably feeding on them.
Jenn