What Causes Yellowish Penile Discharge?

i recently have penile discharge ,yellwish to greenish , when i sqeuze my penis .

i know that is sign for STD but i didn’t go throw any sexual behavior with any person , only masturbation .

my Q is, is it possible to get STD without sexual contact ??
and is there any infection other than chlamydia nor gonorrhea which is lead to this condition ??