Spot algae in my experience is usually the fist indicator of lack of co2 or excess of light, however you want to call it. Decrease one or two hrs and clean up, in two weeks you will see if its still a problem.
When I was studying my tank I realized that taking full tank shots was the only way to see if there was a change on growth over a month and one picture every week same day.
Change only one thing at the time and observe for at least two weeks otherwise its impossible to determine what is going on. Compare your tank pictures.
The only way to know if something is wrong is by doing it right at some point. By this I mean to learn a technique that will ensure you get maximum growth and plant health, a fool proof way to do it is by providing more than enough of everything except light. What is everything? Plant mass, nutrients, co2, good substrate. Light then would only be an accelerator of growth rate.
this is why Tom came up with EI. Non limiting factors, Liebig's law. Correct me if I'm wrong Tom.
When you achieve to experience that kind of gardening then you will have something to compare with, otherwise you will never know. From there you can adjust to your needs.
First, master EI with enough co2. How to make sure you don't gas your fish?
Proper surface agitation, no surface scum... In other words Good atmospheric gas exchange.
Temperature plays a huge part in all this, specially because warmer water doesn't hold gasses very well, leading to more fluctuations of O2 and CO2 that are known to cause algae blooms and even fish/invert suffocation.