I've had my Bonsai tree for 6 months now and recently this grey/white fuzzy stuff has been growing on the top soil of my tree. I scrapped it away and left it but it keeps coming back. It also appears to be on the stones in my humidity tray. I've not watered the tree for some weeks and the leaves and trunk itself are fine. But I'm worried it'll get worse.

For context, the Bonsai tree sits on my window sill and it's the winter here.

Pictures are attached.

What you are seeing is calcium. Limestone.

If you have not watered the tree for some weeks, something is wrong. It should be bonedry by now.

It was very dry. What would you recommend then?

EllieJean wrote: It was very dry. What would you recommend then?

I'd start here:
www.gardeningknowhow.com/houseplants/hpg...e-of-houseplants.htm

Also, you may want to get rid of the 'humidity' tray. Sounds to me the pot is standing in water.