From your photo, the tank is beautiful. Everything is thriving except this epiphyte. It wouldn't have the algae if it had everything it needed to grow. There is a weakness. It doesn't have roots in substrate, so it is only getting nutrition from the column. It can't get something. Dunno what it needs. The roots and soil would provide it to top itself off if needed. Do the ones that have roots in substrate have less algae?Some have access to soil, insofar as any roots could reach it if they wanted. Curious as to why you think this may help?
GH is around 5 and KH 1 using RO water remineralised with MgSO4, KHCO3 and CaSO4.
In this case, if it were me, assuming your KHCO3 gives at least 15 ish K, I'd simply drop N to APT complete targets ~ 6NO3 and ~3PO4 ... basically, a 1/3 EI for N, and keep the PO4 high. At the same time, feed a little bit more to fish to get some more urea and ammonia into the system. Keep the lights as is.
If you had high GH, I'd suggest doubling your EI and watching.
Please don't misinterpret this -- lowering your light and nuking the algae in a dip will work. But you may experience longer internodes on other plants, some may get leggy, and leaf size may increase - coloration may also change. There would be no need to reduce your fertilizer with your light - it will become the same situation by staying non-limiting so just pour it in to safeguard the system.
All of this may not happen to a noticeable degree to a passerbyer, but you will notice --> based on your photo the tank is svelte.