Since it isn't metal, you could just clean it out with salt or copper sulfate, and scrub brush or sponge that gunk out of there. I find myself having to scrub clean my waterers about once a week. I spray a little Oxine in there, scrub them out, and rinse them. It should be done more often when using supplements in water. I do this with trough feeders too. I dump the remaining water at night when I lock up birds and rinse out waterers with a high pressure hose, then refill the next morning. I think lots of folks tend to overlook cleaning out waterers daily.
