Those roots were most likely dead before the algae started to colonize the exterior.
With living roots, there would be some algae colonization but not to that extent.
Check the roots. If they are still alive then the algae would be benign and would actually help to hold moisture. Since the plant is mounted, it would need all the help it can get with keeping moisture available.
If the roots are dead, remove them as they are just invitation to more infection.