You can try chopping up the Mysis when it is frozen. That will give them smaller pieces to eat. I've found that Larry's Reef Frenzy seems to be a good mix of sizes for all my fish if you want to try something different.
Another option is Cyclop-Eeze which I find to be crack for clownfish. Although based on its tiny size, I'm not sure if they would be able to get enough to eat if you were to feed that exclusively.