Algaecide is not a good option. If it's effective it could easily harm all your plants and not just the marimo balls. Kill off too much algae and plants at once and you could end up with an ammonia spike that could harm your fish.
It looks like you're using 2 WPG over the aquarium. How long are the lights on each day?
As you've mentioned, CO2 probably isn't necessary with the amount of light that you're using.
Which plant tabs are you using and which liquid fertilizer are you using? We need to know if you're dosing Micros and/or Macros and how much of each. Specific dosage that you are adding each time as well.
Please post specific numbers for your test results (most importantly Nitrates and Phosphates). What you are reading as normal may indicate a course of action for us. For instance in a planted aquarium you don't want 0 nitrates, although this could be considered normal.