Sounds like oxidation.
I was at a Marine store yesterday (Boats US/West Marine) talking to a guy about a pre-wax cleaning for the gel coat. I had a slight discoloration/oxidation happening.
There are different products you can buy with varying abrasiveness.
He liked the Dolphinite products, but there are others.
I bought Dolphinite Wax Cleaner for medium oxidation with a medium abrasiveness. It worked well for what I had. If you've got a real chalky oxidation you're gonna need something with a little more abrasiveness. Here's a link: http://www.boatus-store.com/webapp/wcs/stores/servlet/SiteSearch?navbar=Y&keyword=&searchDeptId=4&search Manf=DOLPHINITE&image1.x=14&image1.y=15