I have a dicsonia antarctica tree fern in a pot. It appears to be growing well and has produced fronds since planted in its pot. However the soil has this covering. What is it and is it harmful To the tree fern.
This is Liverwort. It is not necessarily harmful to the soil. In fact, it can have some benefits. Liverwort helps to retain moisture in the soil and can provide a protective cover for the soil surface. In this case, both will prefer such moist conditions.
I would be sure to feed the container with a complete liquid fertilizer once a month or once every two months or so to avoid competition for nutrients.
These articles will help:
https://www.gardeningknowhow.com/ornamental/foliage/tree-ferns/planting-tree-ferns.htm
https://www.gardeningknowhow.com/ornamental/water-plants/liverworts/liverwort-information.htm
The trunk of my fern tree looks diseased and has small dark brown pellets on the trunk. What could this be?
There is a chance that it is not getting enough water on the trunk. They do tend to need moisture on the trunk to keep it healthy. On the other hand, care needs to be taken to avoid overwatering, as well. The trunk and soil needs to be slightly moist, but not wet or dry.
https://www.gardeningknowhow.com/ornamental/foliage/tree-ferns/planting-tree-ferns.htm