I use Heico Permawash to clean my trays. Works really well to clean the darkroom floor too if you're ever inclined to do so...
You probably don't need anything fancy. I just use the above if it's really bad. Normally I just scrub it with a piece of filter pad that goes in my basement air exchanger (it is pretty much inert material).
You can click the button 'Go Advanced' and attach the photograph to your post if you want. What's nice about that is that there is no risk of broken links in the future.