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It's most likely green dot. Just get rid of the bottom leaves that have it, they won't disappear unless you get horned nerites or removal of the leaf.
Also your stauros look a little beat, I find they are serious root feeders as I've had around 300+ stauros where most roots reached over 6" long. Your EI and gravel substrate might not be enough, I recommend root ferts or a plant substrate.
This attached image was my old carpet, went on vacation for one week and carpet yellowed and turned skinny without fertilization - very heavy feeder.

 

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Thanks. It does look like GSA. Seems to be all contained on the plants, none on the glass yet.

I am dosing via EI, it's odd that I would have a shortage of phospates. My photoperiod is not too long and I have enough CO2 in solution according to my drop checker (indicator matches the reference almost exactly).

I added some root tabs under the plants, hopefully that will help them.

I don't need it has much to do with you co2 or lighting (what is it anyways?) where I think the problem might be for you stauros is your lack of fertilization in the root base. Is the tank dirted? Stauros should be growing compact, if it's lanky and stretched you mostly have a deficiancy. If the plant is healthier it would less likely have algae problems.
Issue could be your light and/or fertilization.