Spot is just that, a spot............nothing more, nothing less and is not an issue.
When growing in soil like you are, never water to a fixed schedule like 1 litre every 2 days.
Always water the entire pot until some comes out of the bottom of the pot (run-off), then do not water again until the top 3cm or so have dried out again, be it one day or 10 days.
As 0011 said, cannabis prefers a wet/dry cycle and hates being constantly wet (in soil).