Perhaps, the plants in your tank benefited from the Excel solution and
starved the Algae to death?

I noticed a similar occurrence when I added some lawn fertilizers to my
tank. The plants grew very rapidly and the brown slime that used to be
in my tank disappeared. I can jump to the conclusion that the lawn
feritlizers killed the algae but I rather not do that. I don't deny
that there may be a causal relationship between the brown slime's
disappearance and the addition of the lawn fertilizer, but I won't go
so far as to assert that the lawn fertilizer killed the brown slime.