Shot a very expired (1995) roll of fomapan 400 (totally unknown storage, but almost certainly not refrigerated/frozen) and developed in rodinal (1/50). Looks like mold maybe? Any chance this has to do with bad development of some kind (my first time using rodinal but never seen anything remotely like this).
Albeit most likely due to interaction with the backing. When I've seen film that looked like this, I wound up concluding it was due to humidity (can happen if frozen film is unsealed while still cold) and/or fungus growth on the emulsion.