If you have any tools that can run water at high pressure, they can clean the overlapping area. You could also use a hose with high water pressure.
For this, you should first use soapy water. Any cleaning liquid that can break down the build-up quickly is highly recommended.
You could also use a mixture of white vinegar, lemon juice, dish soap, and baking soda for convenience. That will remove the grime from the surface quickly. Get the liquid through the crevice and let it sit for a while.
Then using the same water pressure and clean the soapy liquid out. Although this is not a great method to clean your shower doors, you can use this method when in a hurry.