Nobody wants slimy mushrooms so don't clean or wash them until you're ready to use them. Baby bella and other grocery store mushrooms are usually pretty clean so a quick wipe with a paper towel is usually sufficient. If the mushrooms are dirty, you can give them a quick rinse in cold water. Don't soak them in water and dry them with paper towels after rinsing. Rinsing them can make them slippery, so another option for cleaning is to use a mushroom brush or kitchen brush to get any soil off before giving them a final wipe with a paper towel.