The old original filter has disintegrated  so much it has deformed to "hug" the actual coil. 

Flushing the toilet daily is enough to prevent any mold from growing in that space, so I suspect it's likely a disintegrating flapper seal or something inside the tank.When someone complains of mold, I share this EPA Guide to educate them.

Old houses have cast iron pipers that will disintegrate (because of age and/or foundation shift).

A leaning wall means it's disintegrated in corner/s.

Old houses have cast iron pipers that will disintegrate (because of age and/or foundation shift).

A simple snake out of a drain most times can result in the snake breaking through an already disintegrating cast iron pipe...as you try to replace the portion of the drain pipe (usually in the wall after you find it) you wonder how the water from 4 apartments drained through a 1/4" hole in a 2" pipe........I think you get the picture.

You never know.I feel uncomfortable with the idea of owning something that could literally disintegrate or "go slum" under my own eyes.

Cast iron becomes brittle and disintegrates, there is no patching or lining.

Hard to see it as a buyers if demand doesn't completely disintegrate somehow.  

Old houses have cast iron pipers that will disintegrate (because of age and/or foundation shift).