This sounds like biological growth of mould, mildew or algae and is often the result of damp conditions. To clean this and help prevent occurrence in the future we recommend the following. Scrape off as much surface growth as possible with a paint scraper, stiff brush or broom. Use Barrettine Mould and Mildew Cleaner to clean off any remaining growth, this product will also kill off any remaining mould or algae spores in the surface of the wood. Two treatments may be required if the log cabin timbers are heavily soiled.
For new or untreated log cabins, follow the above then treat with 2 coats of Barrettine Premier Wood Preservative which will help protect the wood from future biological growth and woodworm etc. Finally treat with Barrettine Log Cabin Treatment which will protect the wood from moisture and water ingress.