What is wrong with my Mata Mata..?

My first guess (and that is all that it is) is intestinal impaction. Do you have gravel in the tank? Certainly, make an appointment to see a reptile vet, ASAP. You can find lists of them at anapsid.org and herpvetconnection.

First off I'm extremely impressed that you have a Matamata because they're so endangered.

Now, do you have a filter? Certain turtles don't react well to tap water in different areas, such as tap water in Texas is known as not being turtle friendly. Also the water level is really important, make sure you have enough water about 6 inches high. If he has white spots on his shell he may have a disease that's equivalent to ick in fish. If so, replace your bulb with a red light. Also, you need to make sure you have the right about of wattage for your lightbulb. 50-75 watts, depending on your turtle's age and size. (Along with a sufficient basking site) I would tone down the water temp too; I know they're an exotic breed, but I would bring it to 80-85 instead. Is there enough hiding places for your turtle? Matamatas like to burrow and hide, hence their amazing camouflage.

It could be a hundred different things. I hope this helps though.