When you see a person with low self-esteem or low self-confidence the first thing you need to do is pay them a compliment that will help lift their face off the ground. It may be the only compliment they get all day and it can be very important to them. So it could be as simple as “you have a beautiful smile” or “what a great sweater your wearing”. Anything that would make them feel special or good about themselves. Then you can measure their mood and take it from there.
I wouldn’t necessarily speak about a person’s mood, just keep giving them positive encouragement and posiitve signals. If they speak about it, just keep giving positive reinforcement without judging.