I am new here so I wondered if you could help. I am growing Moneymaker tomatoes and the seedlings are now about 4inch tall and looking ok. The problem is that the compost in the pot keeps getting some white/grey fur/powder stuff on it. I tend the just rub the compost with my finder to get shut of it but after a few days it comes back.

Could you please tell me what to do.

I have heard the same problem before. I think the reason is that your pots are too wet and not enough air circulation.

Try to keep the top surface on pots dry. Pour some pearlite on top of pots to diacourage the white flies.

Another thin is to water with camimile tea.
Last resort is to get houseplant insecticide that kills white flies.

However since you are already using it then for it to work you have to control your watering of the plants VERY carefully and you have to increase the air circulation in the area dramatically with fans.

Watering when using compost in small containers often has to be reduced by at least 1/2 and sometimes more. It can't be done by the appearance of the surface. You have to stick your finger deep into the soil to the root level to determine moisture levels. When in doubt, do NOT water. No misting and bottom watering only will help.

Air circulation is vital to suppressing mold and fungus growth. Small fans in the area delivering a gentle breeze across the soil surface and lightly stirring the plant leaves will help a great deal.

Dave