Certified GKH Gardening Expert
This crust is normally a salt layer. It forms if you are using a rather hard water or if the fertilizer is a bit too much. I would gently scrape it off and dispose of it. Add a little new potting soil to the pot to replace it. If the plant seems to be doing fine, this is all you need to do. If the plant has looked a bit weak, you may want to use a distilled water or cut back on fertilizing.