I would visually rate that system at 2 - 3 years old is that correct
it's not that nutrients are strongly tied to low-level algae growth, its popular online to claim so, but doesn't really pan out in gigantic algae correction threads. your sand bed has some minor accumulations in it, no worse than mine, no grazers matched for your invader, a few years maturation in this condition, and you have low level algae growth.
there is specific work you can do to make this go away in about a week and not come back per 500 examples in our thread.
it is also an option to try and handle the algae only through the water and deal with the variables that will lend