Hate to be the messenger of bad news Col but from the appearance of the photos and your description of the location "between" the elements I would say it is a fungas/mold - the filament structure/pattern is key characteristic
It can be fixed however via a variety of options - you could try some UV or sunlight - if this doesn't work then either take the cell apart (if you fell comfortable/confident/competent enough) otherwise off to a professional optics outfit would be able to disassemble and remove it.
Anyhow, sorry to learn of this situation and in light of this you might need to invest in some more dessicant or rice (uncooked
).