noun
grey moustache; gray mustache
Specifically refers to a grey or white moustache, often associated with age.
祖父は立派な白髭をたくわえている。
My grandfather sports a fine grey moustache.
noun
white beard
Refers to a white or grey beard, typically full facial hair rather than just a moustache.
サンタクロースは長い白髭を生やしている。
Santa Claus has a long white beard.
Compound of 白 (しろ, 'white') + 髭 (ひげ, 'beard/moustache'). The reading しろひげ is a straightforward kun'yomi combination.