How to Break Apart an Unbreakable Humidifier to Clean It
Put the humidifier in your garage or outside. Put a 50/50 vinegar/water solution into the tank and turn it on. Let it run until the solution is gone. This will help dissolve the deposits. If that doesn't do a sufficient job, try using something stronger like 100% vinegar or CLR.
If running it with both cleaner solutions doesn't do the job, take that top lid off, place it in an aluminum pie pan (or similar), then pour either of the cleaning solutions into it until it's covered. Let it soak for several hours.
Once you've got it sufficiently clean (with either method), run a couple of tanks of plain water through it while it's still outside to get rid of the chemical smells, then return it to regular service.