Nice tank and fish!
I'm sure other more informed people here can diagnose the brown algae/diatoms or whatever it is. When I had similar issues, I would rub off what I could and go from there. My tank also began to look much better (as a whole) when I cut my light period from 9hrs to 6.5-7hrs.
Excel, Metricide, Cidex,... are all essentially glutaraldehyde in various percentages. You can usually buy metricide (14 or 28) for $16-25/gal and have "Excel" cheaply. You can either add water to it and dose at the Excel recommendations, or dose less at the strength out of bottle. Either way, if you dose this... you are dosing what people refer to as "liquid carbon". Whatever you do, be sure you throw out the bottle of activator that comes with Metricide. It is not needed (and will kill your fish).
Don't let the price tag of pressurized CO2 scare you off if you really would like to do it some day. If you're handy and into DIY, you can put together a nice pressurized setup for well under $400 including the tank. I keep meaning to do a little build log on a budget dual stage reg setup with selonoid that I have for under $100 invested. If you're patient and enjoy watching for deals, tosssing an odd $20 here and $40 there on eBay will get it done. Glutaraldehyde still compliments pressurized CO2 so you can keep that going with it if you believe it's working for you.
Using Osmocote (or Dynamite or whatever brand) in place of root tabs is a great idea too!
I never go full dark as when it's night time I have my Lunar blues in I don't know if that's considered a light cycle?
Not sure, but if you google "blue leds brown algae", you can read for days. All my lights go off at night, but like I said, there are much more informed people on here than me!
Edit: interesting excerpt from some of the google reading...
Changed lighting scheduled to a 4-2-4 (4 hours on - 2 hours off - 4 hours on – Blue lights on).
Results: Small decrease in brown algae growth
Normal Light schedule but blue lights off
Results: Large decrease in brown algae
Normal Lights on 8 hours – Blue Lights on
Results: Large INCREASE in brown algae
Normal Lights on (no dim) 8 hours – No Blue Lights
Results: Unable to find any brown algae in my tank."