Some cats get more eye boogers as they age. When it is a hard crust it is just an accumulation of the wet tears that dried up and not wiped. You may need to wipe more often. I wipe eyes 3 to 4 times a day, more if necessary. The dark color under the eye happens in very light colored animals and is not a worry. A lot of people do not like the looks of it but there is not much to prevent it from happening.
If one eye is watering more than the other he may have poked it or has a clogged tear duct, but if it doeas not water REALLY bad, I'd just wipe it more.

Now his coat is probably suffering from the diet food. It will not have enough fat in the diet to keep the "sheen" in the coat. If they are not overweight you may want to give them a fattier food. Or at least at one meal.