It looks to be a bit of light burn to me.
has any of the leaves been wet when the lights on?
the rest of the plant and it upper growth is looking healthy.
I would not worry to much about it as its not spreading.
When will you be starting your feeding of nutrients?
and what kind are you using? crystal/liquid slats or dry amendments?
Let me know if you need any more help with this, you can use the DM feature.
Your inbox is the little green arrow at the bottom right of the screen.
Cheers,