I have some fungus between sistered joists under my deck. It is white and looks like spray-foam insulation. Unfortunately, it isn't. It seems like an important joist, since it was sistered and it supports the top of the stair. It is about 7 feet off the ground and I can walk underneath. This is a very very damp part of my house, since I am adjacent to wetlands. I'd hate to pull up the decking and replace this joist, since it probably means I have to take lots of the deck apart. There is also one other joist with similar issues. Any advice on what to do next?