Surface scum is related to CO2 deficiency as well as nutrient deficiency. So is hair algae. Nutrients don't cause hair algae, so dropping down when you saw the algae was an inappropriate action. Try adding more CO2 and more nutrients as well as increased water changes. Doing half EI is like eating half your meals or like earning half your salary. Likewise, the disappearance of the hair algae is not related to you cutting of the nutrients. This is an optical illusion which causes many to draw a false correlation between nutrients and algae. I know this sounds obstinate and it's easy to be skeptical when you are looking squarely at your algae and shaking your head with dismay.
When a plant faces a deficiency it calculates that it cannot sustain the same biomass and continue to survive. The result is that it cannibalizes itself. Weaker leaves and stems have important compounds withdrawn and sent to stems and leaves that are more likely to survive. As this breakdown occurs the proteins, enzymes and lipids/fatty acids which cannot be reconverted or transported efficiently (i.e the energy required to reconvert or to transport is too high) are jettisoned into the water column. This results in the protein scum that we see at the surface.
Add more CO2 - lots more, (or supplement with Excel/Easycarbo) and dose appropriately. Within 3 weeks you should see the scum start to disappear and your plants will look better as well.
CO2 and Nitrogen are coupled so that an increased uptake of one leads to a demand for uptake of the other. This is not surprising since both are required for photosynthesis. Nitrogen is used to produce the chlorophyll which in turn uses [light + water + CO2] to produce carbohydrates.
This is why playing around with half this and half that more often than not creates problems. Ensuring that your plants have more than what they need, especially Carbon and Nitrogen, will give you a better chance of success.
Cheers,