It could be salt build up, or as in my case the dreaded Mealie bug. Mealie bugs, if you've never experienced them, are white and have a fluffly sort of look to them. They start at the base of leaves, and the stem, hide in the roots, and in my experience when they were really bad extended down the leaf, to the tip. I'd rather have the mold problem, I think it would be easier to get rid of...
Good luck and I hope its not too hard to get rid of for you.