Excel is providing a carbon source, flourish is providing trace minerals but you have no ammonia/nitrogen source for food. The Excel is going to make the moss want to grow faster and need more nutrients and nitrogen food but without anything nitrogen source, that could be effecting it.
I don't see any reason to keep changing the water. Water only needs to be changed when there is a buildup of ammonia/nitrite, etc and you're not providing a source for that in the first place.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps