Mustard algae typically thrives in warm water, but cooler water pools with very low to no chlorine invites this type algae.
Other factors include but not limited too:
Inadequate pool maintenance such as pool (POOR/NO) water circulation
not brushing regularly
not chemically balanced
not periodic shocking
heavy swim loads
dogs
Inadequate filter cleaning
pool run time not long enough
dirt areas around pool
dirt blowing (monsoons)
foliage from trees and bush
out of range Ph/Alkalinity
high phosphate levels
high calcium levels
high stabilizer levels
damaged surface area
chipped
rebar stains
etc
To help prevent the growth of mustard algae in your pool
Maintain proper chemical levels
shock pool regularly
Brush pool minimum once per week preferrable with a combination brush that contains both nylon and wire bristles on PLASTER AND PEBBLE TECH SURFACE ONLY
use nylon brush on vinyl and fiberglass surface (lightly brush)
backwash and clean filters regularly as recommended
wash and clean pool toys regularly