I recently found an old Pioneer VSX-3300 receiver in my basement. It's got some issues with the left and right channels cutting in and out, but otherwise it's fine. I opened it up and took a look, and found these greenish-yellow splotches on the underside of the PCB:
I've done some research and I believe that it's solder flux, but I'm not sure about that (I feel like it could also be oxidization). I'm inexperienced in electronics repair, so I have no point of reference to fall back on. If anyone could confirm what this stuff is and advise me on how to safely clean it off, I'd appreciate it! Thank you kindly.
Also, if this is a totally inappropriate topic for the DIY forum, I apologize profusely and effusively for my ham-fisted intrusion. Have a nice day.