The kind of smoky or hazy aquarium conditions you describe are most likely due to a bacterial bloom. These blooms of bacteria are fairly common in newly established aquariums and are the result of vast numbers of bacteria multiplying unchecked, their growth fueled by excess nutrients in the water. The sheer numbers of these bacteria is what turns the water smoking or hazy, and the metabolic activity of so many bacteria can reduce the dissolved oxygen levels in the aquarium, making it more difficult for the seahorses to breathe.
If you have a lot of Caulerpa macroalgae in the aquarium, a vegetative event during which a mass die off of the Caulerpa occurred can also turn aquarium milky white in a matter of moments, and precipitation from improperly mixed saltwater can also turn aquarium cloudy, but I suspect the smoky or hazy water is due to a bacterial bloom and subsequent drop in the dissolved oxygen levels in your seahorse tank.
I suspect that you’re probably getting some transitory ammonia and nitrite spikes in your seahorse tank following heavy feedings, and that the cloudiness or haziness of the water is due to a bacterial bloom fueled by excess nutrients in the tank. Overfeeding or scatter feeding frozen Mysis can contribute to such problems by promoting wastage and spoilage.
If your seahorse tank has only been up and running for six weeks, that’s barely enough time to cycle a new marine aquarium from scratch. Your aquarium could have used more time to mature and for the biological filtration to become fully established before it was stocked. That’s a problem that only time can correct, Kathy, so you’ll have to rely on partial water changes to keep the water quality in the acceptable range while the biofilter in your aquarium matures. Removing the biowheel from your aquarium also removed a significant amount of the biological filtration ability for the aquarium, which may have further exacerbated the problem.
In the meantime, I would recommend increasing the aeration and circulation in the smoky aquarium and adding a good brand of activated aquarium (i.e., low ash content and phosphate free) to your canister filter. Feed the seahorses very sparingly until the water quality has been corrected and the aquarium is crystal clear again.
Best of luck restoring your water quality and getting your seahorse tank back to normal again, Kathy.