Diagnosis
That's a mix of toothpaste, saliva, and water minerals building up in the gap between the brush head collar and the handle. It's a common maintenance spot.
How to Fix It
You need to break up the gunk without flooding the internals. Here's the safe method. 1) Remove the brush head. 2) Run the toothbrush under a thin stream of warm water for 10 seconds to loosen things. 3) Turn it OFF. 4) Use a dry, stiff-bristled toothbrush (an old manual one) to scrub around the metal shaft and inside the collar area. The key is mechanical action. 5) Rinse again and shake off excess water. 6) Always store it upright, not lying down, so water can drain out.
If That Didn't Work
For stubborn white mineral deposits, dampen a cotton swab with white vinegar (not bleach!) and gently wipe the metal shaft and plastic collar. Let it sit for a minute, then scrub with the dry toothbrush and rinse. The mild acid dissolves the scale.
Heads Up
Never submerge the handle or run it while fully under water. Don't use harsh cleaners, alcohol, or hydrogen peroxide on the plastic—it can crack or discolor it.