Problem Summary
Visible green, black, or mustard-colored patches on pool walls or floor.
Visible algae patches on walls or floor are further along than the ‘slippery walls’ stage and usually need a full shock/brush treatment rather than a spot clean.
Possible Causes
Chlorine has been insufficient for an extended period
Established, visible algae colonies typically take more than a day or two of low chlorine to form.
Poor circulation reaching that specific area of the pool
Algae patches often cluster in the same low-flow spots repeatedly, pointing to a hydraulic dead zone rather than a one-off chemistry lapse.
Diagnostic Steps
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Note whether patches recur in the same locations after cleaning.
Recurrence in the same spots points to a circulation dead zone; scattered, one-time patches point to a general chemistry lapse.
Solutions
Brush all affected surfaces thoroughly, then shock the pool
Addresses cause: Insufficient chlorine over time
Use the Chlorine Calculator for the correct shock dose, and keep the filter running continuously until the water and surfaces are clear.
When to Contact a Specialist
If the same wall areas keep growing algae after repeated correct treatment, have the pool’s circulation pattern and return-jet placement reviewed — a genuine hydraulic dead zone usually needs a design fix, not more chemical.