While repotting cactus if you observe that some roots have a white, ash-like material on them, with patches of it in the compost then it could be the root aphid, a sap-sucking insect like greenfly but living in the soil and feeding on plant roots. It can multiply considerably without detection until the plants are nearly dying.
Suspect the root aphid with any plants which, despite having the right food, water, light and warmth, are
growing slowly or not at all and have a sickly grey or yellow look.
Infestation can be confirmed by turning the plant out of its pot and examining the roots and compost, when you will see the symptoms you mention. It is most likely to occur in dry soil conditions, so cacti and succulents are particularly common targets.
Treat affected plants with systemic insecticide according to the maker’s instructions, or wash all the compost off the roots, wash the roots thoroughly, and then repot in fresh compost and clean containers.