Bear in mind that the omega 3 fatty acids and lipids in mysis shrimp and other frozen foods, including ours, begin to oxidize immediately upon thawing and degrade every hour the food is thawed. While many people do it and it likely won't hurt your fish if you put some in the fridge overnight, I wouldn't call this a best practice. We add buffered ascorbic acids to our foods to combat this breakdown, since it is a natural antioxidant.
The "best" things you can do to maintain the integrity of any frozen food is:
Do not let it thaw on the car ride home from the LFS. Plan a head and take a cooler with ice packs. (Thawing and refreezing causes cell walls to breakdown and can cause the lipids and nutrients inside the organisms to leech out. You want those nutrients to stay inside the mysis, krill, brine etc.)
When you feed, remove the food sheet or cube etc from the freezer, break off what you need and get it back in the freezer asap. (Nothing causes your food to get ice crystals and freezer burned faster than the condensation formed in package from the daily freeze/thaw if it is left out on the counter each night for ten minutes.)
When you buy frozen food place it in a second ziplock freezer bag as soon as you get home.
If you buy food you plan to store for 4+ weeks, place it in a second ziplock freezer bag AND wrap it in a cut up brown paper grocery bag. Then place it in the coldest part of your freezer so it is not exposed to the constant opening of the freezer door, if possible.
The biggest factors in keeping your food fresh and packed with nutrients is fending off moisture, and freezer burn from daily exposure to air.
I would not recommend thawing any frozen food product, rinsing and refreezing it.
Just my $.002
Larry