hitchhikers don't indicate water qual issues they are import/non qt issues
that isn't a green algae it might even be some type of bryozoan hard to tell from the pics. brown is a fine photosynthetic pigment for visual detection so it easily could be an algae as well, but not a common strain.
see how your live rock, with pure algae excluding coralline, is algae free? that stuff is concentrating where its allowed to get a foothold...since the nutrient in question water touches everything if it was a nutrient problem id have to expect to see it everywhere.
it colonizes where it can, and that's correct for the tank. to be algae free is abnormal.
though algae might be beaten by nutrient reductions in the tank, in the wild that isn't the case along with most algae, its grazing that keeps it in check. in study after study after study when you box or cordon off any pristine/algae free reef (which we all link to great nutrients) from all grazing, algae of all kinds develops off the resources still available in the water and from the light, they're that good.
So by that standard, the fact that nutrients are the only way to deal with any algae certainly doesn't line up with nature although we like to claim that a lot, every time, for every tank without individual weight. I see nutrient control in aquariums as a fine preventative effort but not the final say like hand removing or lucky grazing can be