If a Haworthia is suspected to have pests or diseases, it is a good idea to quarantine it before careful inspection and treatment. This is especially important for new acquired plants.
At first remove the old soil and wash the roots in water. Always clean as much of the soil out of the roots as you can. Some roots are bound to be torn off in the process, but it’s unavoidable and the plant will be fine. Meanwhile, look for signs of pests. They tend to hide around the base of the plant (where the leaves meet up with the stem). Treat visible pests if there are any.
Next, dilute insecticidal soap with warm water, soak the whole plant in it for several seconds, and keep the root and the base in the soapy water for the next 30 minutes. This kills any mealybug eggs and root mealybugs that may be in there. Then gently rinse the plant in running warm water.
Repeated the above soaking process, this time with 1% hydrogen peroxide (and dilute with cold water if needed) for 15 minutes. This kills any plant pathogens including fungus, and will promote faster healing of any roots that were damaged.
Once done, let the plant sit out in a dry, warm and shaded place before planting it in a fresh soil.