Paul...corrosion! Most likely caused from the wet carpet wicking into the alum base and the stainless screws holding it down cause dissimilar metal corrosion...basically forming a battery and eating your pedestals.
Take them off and take the scotchbrite pads to them, or aluminum oxide sandpaper. Do not use a wire brush or wheel as it will imbed metal particles in the alum and they will rust. Clean it up the best you can and then etch it with a good alkaline cleaner or phosphoric acid. I'd then alodine it, and if you were close I'd lend you a bottle, but you can probably skip the alodine. Now as soon as they are dry from the rinsing off of the etch/acid get a coat of epoxy primer and then a coat of paint on them. Pick a colour that matches your carpet/side panels or something so it looks factory install.