gdup is right, most of this type of problem is from either rust or sand. Be sure to wash your boat down with fresh water EVERY TIME you use it and even if it sits on land, either inside or outside in a salt air environment you need to wash it.
Also, I have found that every 3 or 4 times after you wash your boat, if you spray WD40, (or a product that I like called "Corrosion X" which can be purchased at many boat dealerships or West Marine), on ALL your stainless then take a rag and lightly wipe it off, it will stop most of your corrosion issues.
Flush out your cleats, if possible take an air hose and blow it out, then spray WD40, Corrosion X or a similar product down into the cleat then work it up and down several times being sure to push in the middle of the cleat. That should solve your problem.