Something is precipitating out of solution or its algae. Is it on the entire container or settling on the bottom? If it's the entire container it's more then likely algae. If it's settled at the bottom it's precipitation. Whether the precipitation is coming from the container it's being stored in (plastic) or the powder he is using isn't 100% pure which means it has impurities in it. I couldn't tell you what the precipitation is coming from but can definitely tell you that is what it is if it's settling out.
Do NOT shake the container up and mix it, it's not something you want in the tank. If it's algae use a different container and mix new solution. Put this new solution somewhere that it'll see very little to no light. If it's settling precipitation the solution is still good to use as long as the method he is drawing the solution out (i.e. Doser) is drawing the solution from above the precipitation. When he can't draw solution from above the precipitation any longer. Mix new solution either in a new container or clean the old one out thoroughly.
