Tumbling stones will clean/smooth just about anything.I had the same thing. I cleaned it by accident. I needed to polish up some small disks of metal so I dumped out the media. Put in some ceramic stone I got from HF some water and a little dish soap. Ran it until my disks were good an boom tumbler was clean.
That's all I do, water and paper towelsI just rinse mine with water and wipe it out. If the build up is stuck to the plastic, the suggestion about turning it on with a little soapy water in the bottom sounds smart.