Mold growth normally appears as small blackish or grayish specks in the electrode storage solution, and it can also stick to the walls of the bottle and the sensor tip. It does not affect the accuracy or the response of the sensor, but it should be cleaned. Rinse the bottle and its cap with warm soapy dish detergent. If it still persists, it may be followed by a brief (2-3 minutes) soak of 1-2 drops of grocery store bleach and a few mL of distilled water in the bottle to kill any remaining bugs. If you have persistent mold growth in the storage solution bottles, the pH storage bottle and lid can be cleaned with straight household bleach as long as it gets a thorough rinsing afterwards to remove any traces of remaining bleach.

To prevent mold growth, it is a good idea to replace the pH electrode storage solution with fresh solution once during the school year (between 1st and 2nd semester is a good time) and one more time before storing the sensors for the summer. The recipe for the storage solution can be found in the sensor’s owner’s guide (100 mL of pH 4 buffer solution + 10 grams of solid KCl + preservative). The preservative can be purchased from Flinn. Alternatively, we sell the storage solution in bottles of 500 mL. See Where do I get pH or ORP Storage Solution? (Buffer Storage Solution)

To make sure your probe is working properly, calibrate it: How do I calibrate a sensor in Logger Pro?
If you are still having problems: The pH electrode I have been using no longer seems to work or is slow to respond. Is there anything I can do to fix it?

Related:
What is the best way to clean the glass bulb of the pH electrode?
Do any of your sensors require special storage?
The pH electrode I have been using no longer seems to work or is slow to respond. Is there anything I can do to fix it?
My pH Sensor has mold inside the shaft. How do I remove it?