I think that is mealy bug. Is it sticky?. My jade plant had it. Used cotton buds to clean it off but then used a cotton bud to apply a bug killer. so far so good with new clean growth occurring.

Reducing dampness & increasing air circulation may help too. If it's just mildew rather than bugs, remove the pebbles & wash them before letting the soil go dry.
You can spray the plants & soil surface with 3% hydrogen peroxide from a small misting sprayer. Hydrogen peroxide should be for sale in your local chemists but the strength percentage is important, much stronger than 6% & it's more of a herbicide!!

Will try this tomorrow many tks.

Barbara is right, it is mealy bugs. Use a small brush or failing that a bit of cotton wool to dab every white bit with methylated spirit. Instant death to mealy bugs and no harm to the plant. Easy and quick.

That works, you can also use isopropyl alcohol, the kind you find at the supermarket for topical use, and a cotton swab. However you'll still have to find out why you got mealy bugs in the first place and how to prevent re-infestation.

In this country at any rate all you need to do to attract mealy bug sooner or later is to have some cacti and/or succulents, though they do attack lots of other things as well.
You can get it on the roots too, especially if the plant is rather pot bound. Best check. Meths works on the roots too but can be difficult to apply if allowed to get a good hold, and the inside of the pot needs treating too if that happens.

Malathion used to be used very effectively but its off the market now.