Certified GKH Gardening Expert
This appears to be a mild fungal infection. With some treatment, this will be gone quite easily.
The next time that you water, mix 1/4 peroxide to 3/4 water, and water this in. Do this once, and then once more after a week. This will kill off the infection. Old leaves will not recover, but it will stop the spread.
To prevent this in the future, make sure to only water once the top 2 or 3 inches is completely dry if there are drainage holes to allow water out.
If there are no drainage holes, then you will have to weigh the pot completely dry, and fully saturated. Only water once it gets close to completely dry.
This article will help you to grow tomatoes in container: https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-tomatoes-pots-containers.htm
Thank you! I'll give this a shot tomorrow morning and make sure to keep an eye on watering.