If there is associated rubber dust, then pulley corrosion or alignment are typical culprits. If no dusting, then lets hear what the mechanic has to say.
After seeing the pic RT refers to I installed a March turnbuckle, aka "adjustable alternator bracket" for my Ample alternator on a Lehman 120. Gates makes a small tension gage that is easy to use and the turnbuckle lets you actually adjust the belt instead of hoping it's close when you crank down the bolt.
And it looks cool.
BEWARE!
If the alt has a better 3 stage regulator a single belt will be highly loaded.
This can cause the alt belt to slip at the alt pulley , which makes it run hot.
So hot the bearings can melt the grease and the alt will sieze.
Use a thermometer gun to observe what is hot.,