Certified GKH Gardening Expert
That is a fungal infection. If their return policy will allow you to return this, then it wouldn't hurt. These will get soil infections at the slightest overwatering, or even because of not being able to dry out fast enough.
Should you feel the need to attempt to correct this, the best way would be to replace 1/3 of the water with peroxide the next time that you water. This can help correct infections as long as they are not too far infected.
This article will give you further information on the care of these: https://www.gardeningknowhow.com/edible/herbs/scented-geranium/mosquito-citronella-geranium-plant.htm