If you're doing Excel directly on plants that are out of water, I'd use a 10:1 water/excel mixture. Anything more and you stand the risk of killing the plant as well as the algae..
I should say 9 parts water, 1 part excel. Is that 9:1 or 10:1?
I've used 10:1 and it takes a few days but it does kill the algae.