Interesting b/c I'm not sure how light would get that far down in the stand since it has doors on it. Should I be concerned with algae there? Or just let it run its course? (again, i don't see any in the tank itself)...
My guess is the overflow is not covered and light is coming down from there
So what does black tubing do? Does it just hide what's inside? or Does algae not grow on it?
So what's the problem with having a bit of growth in the drain tube?
In my tank the growth seems to have leveled off as a thin film, no thicker than yours. It is far from obstructing the flow.
I'd leave it as is and don't worry about it.