 moss or algae so i would definitely clean it let it dry really well and then you have to make a decision on which way you want to go i mean in terms of stain you've got solid color stain semi-transparent and transparent i'm a fan of solid color because it lasts the longest but if you want to just keep the natural color of the wood to the extent that's possible because even if you just seal it with clear it's still going to turn color underneath all that then you can do that just make sure that whatever you choose has a good dose of uv protection because that's what really damages the log