Cacti and succulents are not the easiest to get clean especially when dust gets between the spines, you can’t just easily brush or wipe off dust and grime like you can with soft leaved plants.
I have found that using a stiff eye make up or an artist paint brush is the best way to get in between those tight spines on the smaller plants, and using a cosmetic brush and larger ½ inch soft paint brush for brushing dirt off larger plants and large leaves succulents, avoid using cleansing chemicals that could damage and even kill your plants, and avoid using cotton wool wipes and buds as the tiny fibres get embedded in spines as the fine fibres can be very difficult to remove.
When it comes to very large cacti and succulents that are heavily spined and its very difficult to reach those hard to get places with your brush, a powerful water spray pump will work a treat, as the pressure of the jet spray will help to dislodge any dust and grime.
A good tip is to use a mild soapy water to clean succulents and leafy plants, a good natural Horticultural mild soap to use with cleaning plants is PinkSun this is a nice mild soap that is safe to use for cleaning your plants and will not harm the foliage, it is also a good safe soap to use with Natural Horticultural Neem oil as a natural pest prevention and treatment, ( see more in the page on Pests and Diseases HERE )
Here is a video below that I have made for my You tube channel on How to keep your cacti and Succulents clean and dust free :
Clean rainwater is always preferred over tap water whenever possible, this is because the chemicals often found in tap water can often be high in lime and will to leave lime splashes over the plants. I also recommend overhead watering if possible with clean rainwater. overhead watering helps to keep your plants clean and dust free and also is a great prevention against Spider mites too.
In high humidity mold can sometimes grow on cacti. Ferocacti are prone to mold on their areols due to the sticky honey like secretion they often form on the areoles, although this is harmless it can be unsightly. Mold can also be a sign of mealybug as the sticky secretions they leave behind can encourage mold. Mold can be removed with soapy water or a bit of rubbing / isopropyl alcohol.
Here is a video below that I have made for my You Tube channel on how to clean mold on Cactus plants: