Since I took the photo shown in the earlier post, it's gotten much worse. If not quickly removed, it multiplies like wildfire.
I spoke to the manager, and she was very abrupt - she muttered something about "I just tell the landlord", and she was quite dismissive - almost rude. I asked if she knew about the Council service where they will pay 50% of the removal cost, and she was again almost rude in her response. It's as if she just didn't want to know - not her problem. Somebody else's problem of course - and if she does nothing, it will magically go away.