Mealybugs are white-grey and about 4 mm long, common on greenhouse plants and house plants like cacti and succulents. They feed on plant sap and are covered by a white, waxy secretion. Look closely for clusters of these tiny white bugs, especially in leaf axils and other hidden parts of the plant. They leave a sticky honeydew on leaves (a sugary substance secreted by the mealybugs), which could be what you're describing.
You can leave hardy plants outside, where birds like blue tits can help naturally control mealybugs. For small infestations on plants like cacti and succulents, you can try removing mealybugs with tweezers.
Rub the sticky secretions away with soapy water or sugar soap to prevent sooty molds developing.
Spray bad areas with fatty acids or plant oils, as these can help deter and control mealybugs.
If the infestation is severe, you may need to consider destroying the affected plants to stop them spreading.
