Disease, burn or otherwise?

[Catt Mandu]

Cellular damage from under-watering and too much sun is a maybe.

The other thing I would check, take a cotton swab, moisten with alcohol, then wipe the underside / top of damaged leaves. See a reddish color on the towel/swab afterward? If yes, spider mite damage; treat with a miticide (note that controls meant for insects typically will not work on mites, which are more closely related to spiders).

[Ami]

Thanks Catt!
There's no reddish anything, no animal friends, nothing.
I figured that it was just cell damage except it appears to be getting worse... today it's definitely darker and squishier - does that happen from alcohol/peroxide typically? Will it just stop or would it be best to cut off the effected area?