Yanmar sheeves are prone to rust !!!!!!! Check the pulley on the engine fresh water pump - notorious for rusting and leaving a roughened surface where the belt contacts the sheeve. Very common problem on QM and GM Yanmars. Also check the surface condition of the alternator sheeve belt. You can remove these and 'dress' them back to smooth if you have a lathe, etc.; but, its usually cheaper just to replace them.
A good idea when storing the boat long term is to spray on 'Boeshield' on the shiney surfaces of the sheeves ... to prevent rust and pitting.
The second problem may be your alternator (load). If the alternator is working against a FULL load you may be exceeding the belts ability to transfer torque to the sheeves. If the the sheeves are smooth and aligned and you are getting 'dusting' than its time to consider to 'derate'/decrease the alternator output - easy to do with an external 'smart' regulator. An external 'smart' regulator will bring the alternator slowly on line (prevent excessive belt 'dusting', instead of 'hitting it' with a full load from the start.
First off - look at the condition of the wear faces on your sheeves; especially the steel sheeve/pulley thats on the FRESH water pump.