Overdosing Excel will kill a fair amount of things if you overdose by a lot.
It sounds like you have a very new setup. It sounds like the algae in question is diatoms. From what I've read so far (and what worked for me) is just to remove it manually for a while and it will clear up on its own. I think I had it for 2ish weeks then it pretty much just completely disappeared without any other changes.
If I remember correctly it is caused by excess silicates which I don't think is added back to the tank with normal use, so once the algae uses up all the silicates there is nothing left for it to feed on and it dies off.