From my experience, I wouldn’t be too concerned about the brown spot – as mentioned, it will grow out, so it could be a bad stain from cecotrope, or even an infected part of the tooth, but it’s close to growing out and will not be a problem with it. If you see abnormalities at the teeth roots/gum line, that would make me more concerned.
In terms of the teeth, I would say it’s either a chip or your rabbit is starting to chew crooked (can be because of molar spurs or discomfort). Depending on how long you’ve been observing it, it could fix itself, or if you find it’s getting worse, I would consult with a rabbit vet to do a dental in order to have them filed even, as well as any spurs which may exist in the back mouth.
For reference, this is my Wick’s mouth currently. From experience, I know Wick usually gets a molar spur on the top right of his mouth, and once it starts to show, he starts chewing odd, causing his incisors to grind unevenly, making this pointy slant on the bottom incisor. We’ll be scheduling a dental very soon to have it resolved.
In terms of urgency, if your rabbit is eating fine and is acting normal, nothing too harmful with waiting a day or two; however, if there’s a decrease in appetite or signs of pain, then it’s much more important to have it done to prevent damage to the tongue/cheeks/gums.

The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.