Certified GKH Gardening Expert
This sounds like the start of a fungal infection. Be careful not to overwater these, as they are very prone to infection.
For now, you can correct this easily. The next time that you water, just replace 1/4 of it with peroxide. This will kill off anything in the soil and help your plant to recover.
Make sure to only water when the top of the soil is almost completely dry down to about 2 inches. This will keep infections from starting in the future.
This article will help with the care of these: https://www.gardeningknowhow.com/edible/herbs/scented-geranium/mosquito-citronella-geranium-plant.htm