The quick answer
Algae appears when sanitizer and circulation cannot keep up with contamination and environmental demand.
Low effective sanitizer
A chlorine number must be interpreted with stabilizer and pH. Water may be under-protected even when a test shows some chlorine.
Poor circulation
Steps, benches, corners and areas behind ladders can receive less flow. Brushing helps expose growth to sanitizer.
Debris and storms
Leaves, pollen, soil and heavy swimming consume sanitizer. North Texas storms can create a sudden organic load.
Filtration problems
A dirty or damaged filter lets fine material remain. Chemistry and filtration must work together.
Prevention
Test consistently, remove debris, brush low-flow areas and respond early to changing pressure or sanitizer demand.