What could be causing the white clumpy stuff in my drain and how can I effectively remove it?

The white clumpy stuff in your drain is likely a buildup of mineral deposits, soap scum, or mold. To effectively remove it, you can try pouring boiling water down the drain, using a mixture of baking soda and vinegar, or using a commercial drain cleaner. You may also need to physically remove the buildup with a drain snake or brush. Regular maintenance and cleaning can help prevent future buildup.