I've been saying that in almost every post.
...It's not an Algaecide like the way Excel is used. It needs good plant mass to be effective.
But I do feel it's treated that way here. It's the first remedy even before the OP shows pics or even describes his/her setup.
Startup tanks and tanks that are old have two things in common. They both have uncontested organic waste in the water column that eventually breaks down into ammonia and they both are algae targets. If you look at new tanks, even the smallest bit of ammonia causes problems that's why the bio-filter gap has to be bridged with A/C, Purigen, Seeding, Large Water Changes.
Old tanks have a build up over time and even with all the water changes, etc. there is still a build up and eventually the tank is not sustainable. YMWV based on light, load, etc.