This is hard to determine without much information.
So far I have got 6 hours of lighting time
With a new LED light unit.
And you have changed the substrate as well presumably within a short period of time.
Based on this, means you have not given the tank and plants time to settle and establish properly since you've changed the lights AND substrate.
Brown algae is usually something like diatoms which is usually common in newly set up tanks so this would make sense if you keep changing things around in your tank.
But the main thing is, you don't mention how long this tank has been running and how long you have had algae going on and when did you change the substrate and lights before you had algae again, this information may be useful before we can give any sort of advice that may be more accurate and helpful.
One or two more question once you've answered the above query.
Do you dose any fertiliser, be it root tabs or liquid ferts?
Is there any livestock in the tank also?
Am not sure you do have livestock since feeding is mentioned in earlier posts but nonetheless if this is the case then both ituwhetu and Bruce are correct in saying overfeeding may cause some water issues where algae outbreaks can be possible.
But at the moment, it does not appear too bad an algae issue judging from your photos so far.
