Hello
1 Actually, your plant looks quite healthy
2 Don't worry about what you have. The damage is very minor and won't affect growth
3 What you're seeing are slight signs of nutrient burn, and it doesn't look like it's going to cause any more damage
4 If the damage does spread to the whole plant, you'll probably need to flush the soil with some fertilizer to dilute the nutrients
Once again, don't worry, only if the whole thing starts changing crazy colors