Certified GKH Gardening Expert
This can be pretty normal. If you are concerned, you can use a fungicide for preventative treatment. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
It may be worth noting that giving that some extra light will be very beneficial to the plant. 200 watts of horticultural lighting will be plenty if it doesn't get to spend much time outside.
Here is an article that will give you all kinds of information on growing tomatoes: https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-tomatoes-guide.htm