Is the dark green algae slimy? Is it easily removed from stuff? If so, it sounds like
cyanobacteria aka blue-green algae. It can be anywhere from really dark green to blue-green to gray in color IME. From the looks of your nitrates, it seems this may be what you have.
If that is the case, you can do 1 of 2 things. Either do a blackout for a few days....turn the lights out, don't feed, wrap a towel or blanket around the tank. Or you can get some Erythromycin (sold as Maracyn) and dose it for a few days. Either way I'd try to siphon out any algae you can and then do it again after the treatment.
I've used the Erythromycin for several days without any effects on the biofilter and it does get rid of the algae.
More water flow and increasing nitrates usually prevents BGA/cyano from returning. You can dose nitrates (for the plants) via liquid Nitrogen by Seachem or with KNO3 which is a dry fertilzer.
I agree^, the dracaena is not an aquatic plant, but aquarium stores do sell it sometimes. It will be making new leaves and growing taller, trying to get above the water. I'd pot it and it should continue to grow.