I'd maybe try increasing the frequency of the actual meaty food. Perhaps you can try your pellets in the morning and the mysid in the evening and see if that helps. How's the coral doing?
Do you have any corals? Or ever buy used live rock (still wet and alive)? Copepods will multiply like crazy, most very beneficial to your water column and some can be parasitic, they love to hichike. Look very closely and pay attention to a rock, if your water has good parameters and have been you should spot some.