I presume it is not really blue moss. More likely you are talking about the blueing of the sap wood present in the pine? If it is the blued sap wood you will most likely not be able to remove it. Sanding it will get rid of the surface crud, but the true blue is throughout the sap wood.

If it has been under cover for two years it should now be dry enough that all the blueing has occurred and will not continue, you have what you see. Bleach is very harsh and I don't feel it will do much good at this point.

It appears you have a conflicting problem in your query about what stain to use, but want to keep the natural look of the wood? I believe the vast reasoning on using oils and or stains will come down to personal choice. I don't see a lot of reasoning behind oils and even more stain. One place I can see oil use is in a high traffic area where it may get handled a lot. The oil will keep it to a consistent shade. Where if left naked it will pick up hand oils and grime, leaving behind a more and truer natural patina. It is these human traces I find interesting, you get to see our reactions within the building, this is the true natural state.

Perhaps others with more oil experience will chime in.