its lichen. how long have you had the forest planting?
I just acquired these about a week ago.
Where did you aquire this nice composition?
Yes but I am with rockm on this...If you don't like it a toothbrush and a little water should scrub it off. If it's stubborn get the tweezers out!
It's lichen. Some find this desireable on older trees. Won't hurt them and marks them as old. Takes quite a bit of time for this to develop. It will probably die off as the tree lives on in a container.
In my experience, lichen like this are highly dependent on local environmental conditions. Once removed from those conditions, they die off rather quickly.