Here is a good link. Im not that familiar with it so I did some research for you. They CAN have white stringy poop with it. I would not get any "food" medications because he is not eating. And it did say in one sight that they hide if you read it and hang thier head up. So It sounds to me like you need to medicate for it ASAP because all the symptoms are there.
http://fishprofiles.com/profiles/diseases/Hexamita/100016/
Cure:
The infestation is highly contagious, therefore hospital tanks are not recommended and the whole aquarium should be treated. The most recommended treatment from HITH is a drug called metronidazole. Other cures have been developed especially for HITH, so follow the particular instructions with the treatment you buy. The treatment should be repeated (as with most parasitic diseases) to ensure the aquarium is completely clear of the disease. Again, like most parasitic diseases, HITH seems to target sick fish, in poor water conditions. (Although occasionally will attack healthy fish as well). So check water parameters and make sure they are the best they can be before commencing treatment. Often improving conditions alone will help cure the infestation.
Symptoms: The first symptom of slimy, white mucous feces, even while still eating and acting normal. Further signs are the fish hiding in the corner it's head down, head above the eyes gets thin, they blacken in color, and swim backwards.
Hexamita are intestinal flagellated protozoa that attack the lower intestine. Discus and other large cichlids, especially Oscars, are especially prone to Hexamita. As it is a disease of the digestive tract, a wasting away or loss of appetite may be experienced.An effective treatment is the drug metronidazole. A combined treatment in the food (1% in any food the fish will eat) and in the water (12 mg per liter) is recommended. Repeat the water treatment every other day for three treatments.
