A/C Problems... i think

Truck: 88 gmc 350V8. been using the A/C and its been making the truck idle and run odd, now i noticed on the tension pulley is starting to lose its paint and the pulley has always been all black for years, and the belt that isnt old at all is looking worn out and there is some type of oil on the tension pulley housing, see in pics.... what is causing this...