That's
Botrytis Gray Mold and a very advanced infection too. I don't think there is any way you can save a plant that is infected that bad already. I hope you removed those stems with the fruit on them and trashed them in a garbage bag. The first photo shows the gray growth where new spores are being produced and they will spread through the air infecting other plants.
You really have to apply preventative fungicide treatments with Tomatoes, you may be lucky for years not doing it, but sooner or later something really nasty like Gray Mold or even worse, Late Blight is going to get your plants if they are vulnerable.