thanks you for this question. i also know something new today about this . melasma happens due to various reason like   hormonal change in pregnancy as well as due to sun light.

reference link :

https://www.healthline.com/health/pregnancy/melasma-pregnancy

so according to me moon for hormons

lagna and lagna lord as well as planates in lagna for face

and mercury is for skin .

saturn for  looking old or wrinkles .

mars is for red spot . specially mars in lagan.

six lord and sixth house is for rog.

so when all this things got connection , then possibility of this things  may happen .as per  my general knowledge . other  members may give their point of view.