The green substance within tanks is usually just Algae. Algae is typically airborne and thrives with sunlight.
We have a Zero Installation Purifier in the office that we use all the time and no green algae (no direct sunlight). If your unit is in direct sunlight or in a well-lit room, I would suggest moving the unit to some other place (with less sunlight) or covering the tanks with something like a towel to see if that takes care of the problem.
Since the filtered water doesn’t have any harsh contaminants, life wants to thrive in that environment.
Would you please clean the tank once again and try covering the top tank from sunlight for a few days to see if that cures the problem. Replacing the filters might help as well. But there is nothing “green” within the filters, so most likely, it’s just algae.
Replacement Filters: https://puricomusa.com/collections/zip-replacement-filters/products/zero-installation-purifier-replacement-filters-bundle