Cloudy eye can be the result of a secondary bacterial infection caused by ich. Do you see any white spots on him? If not, it could be a straight bacterial infection.
Make sure your water quality is in tip top shape. If you just got him it could be carry over from the other system and assuming your params are good it should recover with frequent feedings and time. If you have had him for a while your water params are off or the hippo has ich and you'll see it soon. I would do a big water change and if available, run a UV and carbon to help polish the water off.
there are other things you can do like treat the fish in a QT tank as well. If thats the route you want to go (I would if you can catch the fish easily) I'll give you advice on what to do.
Another thing to look for would be flukes. Most of the time they are not thought of when it comes to cloudy eye. They latch onto the eye causing a 2ndary infection. If you can catch the fish I would try a freshwater dip( fresh ro water with a ph buffer and matching tank temp) for 3-5min. If its flukes you will be able to see them within the first 2min and usually within 5 they will fall off. Most of the time the eye will heal within a few days after that.