What are the moving glowing balls in slither.io?

They give more mass than other bits of food, which is why they are harder to catch.

Fun fact: Dashing causes the glowing food to magnetize to you if you get close enough.

The stationary balls are called 'Food' and are worth between 1 and 20 length.

The mobile balls are called 'prey' and are worth between 50 and 100 length (but you usually have to spend some length boosting to catch them).