This is sometimes referred to as an optical-mechanical mouse (mouse with a ball).
A mouse ball will pick up dust and debris from any surface it rolls over, or oils from your hand, if it is a trackball type. Dirt which has collected on the mouse ball may also be transferred to the rollers inside. Dirt on the mouse ball or rollers can cause the mouse to become difficult to move or cause unusual mouse movements.
Keep the area where the mouse is used clean, dry and free of dust. If the mouse is used with a mouse pad, keep the surface of the mouse pad clean. Occasionally wiping your mouse pad with a damp cloth will remove any build up of dust and grime.
To inspect the mouse for dirt or damage, remove the mouse from the computer and remove the ball-access slide cover. Check the ball for any signs of damage, such as pits, cracks or grooves, or distortion in the shape of the mouse ball. If none, inspect the rollers inside the mouse for any sign of debris or greasy build-up. To clean the rollers, use a cotton swab or carefully use a small, flat-bladed screwdriver to scrape off the build-up. Use a damp, lint-free cloth to clean the mouse ball. Make sure the ball is completely dry before reinserting it, and replacing the cover.
Don't use alcohol to clean a rubber mouse ball as it may cause shrinkage and deterioration of the mouse-ball material.