It is probably not harmful at all since it is outside. I say "outside" because you used the term "transplanted". Regardless, you can spray the soil with a fungicide (if it is a fungus) or a algaecide (if it is an algae) and that should get rid of it. You might have to do this twice, one week between applications. Alternatively, remove all that soil, rinsing off the roots with water so that there is absolutely no soil remaining, and use sterile potting/garden soil. If you previously used a packaged potting/garden soil, that soil was already contaminated.
