Slimy whey in store-bought yoghurt The yoghurt I bought from the store in a 1 l glass jar has in the 4 or 5 days since opening separated a little from the whey. 
The whey is has gone slimy putting me off breakfast.

On this site it has been stated several times that yoghurt cannot really go off.
However, this has happened a few times with this brand and I'd like to know what is causing it (and if its really safe to eat)

It smells like yoghurt and the packaging says it is made with pasteurized milk.
It's probably thickeners, such as carrageenan or xanthan gum, which are thickening/sliming up the whey. Draining off the whey or stirring it back in should be fine. A brand which doesn't have added thickeners (look for "strained" or "Greek-style" yogurt, then read the ingredients) won't do this.

I've had yogurt go off many times, and slimy whey isn't how yogurt goes off. The whey is nowhere near as tempting to microorganisms as the yogurt solids, so you'll see mold (or whatever) on the solids first, not the whey.