I lost my 4 year starter due to mold recently, and my dried backup failed. The moldy one went much the same way as yours... I had forgotten about it for an extra day after feeding it. I saw a film on it that looked more like a pellicle. It smelled different, but not bad. I took a small amount and started feeding from scratch, hoping for the yeasty beasties to outrun the mold and raise the acidity to kill them. It took the feedings well enough but still felt like it wasn't performing quite the same, so I decided to end the experiment. The dried backups failing were less surprising, I had tossed them haphazardly in the freezer years ago because I never really thought I'd need them...

Anyway, RIP to our boys. #yeastr needs a starter exchange.