That’s rather unusual. I have small bottles of Holy Water that last years without any problems.
It might be that you need to give the jar a good cleaning (I suggest using bleach) or it might be that the source needs a good cleaning. If it’s algae in the water (making it green) that algae has to come from somewhere.
Maybe the algae has taken up residence in the rubber gasket?
Where are you getting the Holy Water? Does it look clean? Is it exposed to the air or is it in something with a lid?