why the white layer is formed in the around the tip of men's penis?

Question

and how can we avoid that physical behaviours

Answer 1

yes. that is smegma

Smegma, a transliteration of the Greek word σμήγμα for soap, is a combination of exfoliated (shed) epithelial cells, transudated skin oils, moisture, and bacteria that can accumulate under the foreskin of males and within the female vulva area, with a characteristic strong odor and taste. Smegma is common to all mammals, male and female. Mycobacterium smegmatis is the characteristic bacteria involved in smegma production, and is generally thought to form smegma from epidermal secretions.

Answer 2

bacterial growth ,bacteria grows in hot and humid places ,one must wash the area thoroughly,and maintain hygiene

Answer 3

it's called smegma and it is more pronounced in uncircumcised men