I keep seeing Excel being suggested as a first line of carbon supplementation and thought I would look a little closer at the product.
I run medium to heavily planted tanks with 12 to 14 hour photo periods with good growth and no nuisance algae. I use Flourish Comprehensive once every week or so and root tabs for swords. I don't believe that carbon supplementation is necessary in any but the higher energy systems where fast growth is desired for whatever reasons.
The ingredient in the product that keeps getting negatively mentioned is Glutaraldehyde (chemical name 1,5-Pentanedial). To me, any aldehyde is suspect when being used for living organisms as they are used as preservatives, disinfectants and various microbiocidal applications.
So why put it into a tank at all? I'll go off on that tangent elsewhere.
Anyway, something is out of balance and with algae, it is most often the lighting so start by dropping it, 8 hours might be a good place to start. It's far easier to reduce something and watch for results than to start adding one thing, reducing another and changing something else, you'll never know what the problem was and may even introduce another imbalance.
I'm not sure about that algae, can it be rubbed off of the leaf at all? It would be easier to do that if it comes off even a bit than trying to kill it... which won't work long term until after you have the balance right anyway.