Certified GKH Gardening Expert
It is likely that they are staying wet for too long. This looks like the start of a fungal infection. I would recommend applying dolomitic lime and wettable sulfur to the soil. This will help kill off any infection.
Let the soil dry out to down to about 2 or 3 inches before watering each time. This will help prevent future infection.
This article will give you more information on the care of Dracaena in general. This is one of many types, and they can look very different, but they all have very similar care, as explained here: https://www.gardeningknowhow.com/houseplants/dracaena/dracaena-houseplant-care.htm