The fork legs are clear-coated. Over time, grit chips this, allowing air & water to reach the metal underneath, starting the unsightly corrosion that you want to get rid of. Happens to lots of bikes. You'll have to remove the coating to clean the metal.

I've used wet & dry to remove the coating and finely sand the castings (put some thick masking tape over your stanchions to protect them), then follow up with Autosol paste. It's a bugger of a job, but the end result is much better than what you have at the moment. Be aware that once you do remove the clearcoat, you'll have to clean them more often, unless you have them re-sprayed. Alternatively, if you are servicing the forks and just have the lowers as seperate parts, you could have them lightly bead-blasted and repainted. Your choice.