Mold Under Floor Tile
Mold under the floor tile is a sure sign of moisture.
The first thing I would like to point out here, is that you never install wood over your existing flooring and then install new flooring on top of the wood.
The old flooring should be removed. It can create problems with mold and fungus in the future.
The picture above is a good example, of what will happen to any wood that is sandwiched between two moisture resistant materials, like linoleum flooring and it becomes damp.
If you look to the right corner in the picture, you will notice the wood, with the mold on it, stops about a quarter of an inch away from the white wall. If you look closely, you can see the old flooring.
If there are any small gaps in between the linoleum tiles that were laid over the existing floor and the new wood substrate, this could allow water to penetrate into the wood substrate, but would have a hard time getting past the old flooring underneath the wood substrate.
This will trap the moisture and could keep it there for quite some time. Now we have the perfect situation for mold to start growing. Dark and damp equals mold.
Home Repair Tip - I don't recommend using any materials or plywood smaller than 1/2" thick when going over your existing sub flooring, for whatever reason.
Anything smaller than 1/2" tends to warp and buckle easier if these materials ever become damp.