So I've been in my new-old place for a few months now and I've noticed that over the past few weeks there's a really bright mint green colour that seems to be bleeding through the cream wall paint at various spots, and increasingly so(primarily at edges like the wall/skirting line or next to trim and doorframes). Pretty sure it's not mould or anything like that. Redecorating was something I wasn't going to be doing for ages yet, but I am planning to do renovation work that will involve fiddling with one of the greening walls, is it anything to worry about do you think?

It may be as simple as there was a quick paint job done to make the property ready for sale? What you could be describing areas where there is just less paint e.g cutting in with a brush rather than a roller.

+1 if you are concerned, clean off a small patch of the top coat to see what’s underneath … very good chance it’s just a bright green paint (or paper) that’s now grinning through. 

I with agree with old paint "grinning"