There is! There's extensive white mould on the right-hand joist, and spots of it on the left hand joist above left of the roots. Suggests there's serious decay in the woodwork.
There is nothing on the other side of the joist. This is just growing on the side of the floor joist and the joists appear to still be structurally sound. The white mold that is visible on the right came about due to a lack of venting in the skirting and that is why I thought this may be some kind of fungus as well. Does anyone have any idea what this might be? It is also notable that this area gets absolutely no sunlight.
Could it be going along the joist under the black paper? From some tree growing nearby? The tree roots can go dozens of yards from the trunk towards the source of moisture.
If you really want to know what the growth is, just pull on it and see if it comes off or is growing through the joist. Roots will be fibrous and must have a source outside the shed. Fungus would most likely be brittle and could have originated within the joist.
