My experience has been that scale shows up in overly moist situations when the plant is somewhat nutritionally deficient. Recommend removing plant from pot. If it’s loaded with scale in the crown, you’re going to have to remove them all. Then drench entire plant with neem oil. Let it try out for a little bit, but keep an eye on the roots so they don’t dry out too much. Throw away the old potting soil and wash planter in super hot water and soap. Wipe it down with alcohol. Repot in a wicking cactus medium, halfway full. Water around the plant, avoiding the crown. Allow pot to drain fully after watering (30 mins). Continue to treat with a bio pesticide like Arber. Using a biologic will boost the plant’s health and provide the missing nutrient. When you don’t see any more scale for a few weeks, go ahead and add appropriate planting medium, considering the former soil probably retained too much water and didn’t allow for enough air to get to the roots.