Is this a leak from my upstairs neighbours? Can I fix this myself(as a not very experienced DIY-er) or do I need to call someone in?

For more context:

This stain/wall disintegration appeared back in February much smaller and has increased since. Because it was dry and I had been away I assumed my upstairs neighbours had maybe had a small leak and it had dried out. I have been meaning to paint over it for a while but realised today it has got bigger than when I first spotted it. I wish I’d kept a better check on it but my life/work schedule has been crazy. If at all possible I’d like to not approach my neighbours about this as they aren’t especially friendly and we’ve had issues before, but if I have to go up and ask about whether they had a leak or if something else is going on then I will.

/r/DIY Thread Link -