This is some minor water staining in an unseen area that's been around for a long time. Ain't nobody used that color green in 50 years.
_maybe _ there is a bit of.rotnon the bottom right wall but can't tell from this.
Are you just planning on putting a dishwasher back in the hole ? What's your goal? Because this is fine and honestly I wouldn't do any of what you are thinking unless part of a much bigger project
(Left Side cabinet)
(Right side wall)