I'm having trouble with GSP on my plants only. I know it has to do with low phosphates and low CO2. It's a 10 gallon low tech with slow to medium growing plants. I've had trouble with the whole balance of nutrients, light, and CO2 (I don't use Excel or CO2) for 9 months. Battling diatoms/brown algae and GSP. Now diatoms are to a minimum and GSP is spreading. I've been trimming plant leaves with algae, but it's still slowly spreading.
1/32 KNO3 1x/week
1/64 t. KH2PO4 1x/week
Flourish Comprehensive 1x/week
Root tabs every 3 months
1 betta fish
40 red cherry shrimp...POPULATION BOOM! (I think it's okay. I read a 10 gal tank can have 100-150 with their low bioload. I don't want the population over 40 though. I'll have to find new homes for them.)
2 ghost shrimp
1 nerite snail
1 mystery snail (poor thing's shell is eroding. Maybe from low KH.)
I had some GSA on my Anubias and other slow growing plants. I heard mention of low phosphate and phosphate/nitrate ratio as a cause several times so I bought a test kit and my PO4 was about zero. Bumping it up to about 1 part to 10 parts nitrate eliminated almost all of the GSA. I have also lowered my light duration and intensity so that may have contributed to the improvement also.
I second the snails but that doesn't answer why. I dont think its too much light. I researched those lights heavily and was going to get two just light you. As mentioned earlier with the duckweed I doubt its too much par.
Pennywort is closest to the light though. Pennywort comes in a couple different cultivars. Im on my second batch and to my its not easiest plants.
How old is the tank? How is the overall growth of the plants?
I think its a balance issue. Most of what you have are slow growing plants. Low par and ferts with no co2. Have you ever checked tds?
Something that you may consider as well is water flow. I don't know how much flow you have, but weak water flow doesn't allow for optimal nutrient and CO2 utilization from the plants. If you're flow is good, I'd up the phosphate a little bit. Sometimes it just takes some experimentation.