Why are there white little dots in the henna?

Because you are tinting with natural henna, it comes from the plant and those are simply the seeds! This is nothing to be concerned about and if you continue to mix the solution, you will notice that it will eventually dissolve. The little "bubbles" or seeds will not interfere with the henna tinting itself, so do not worry. Just give it a good mix around and you're good to go.