Mould on the ceiling

Now this makes me unhappy because I don’t know where to turn. This is in my small room which is under the bathroom above and there must be a leak for this to happen! The person who lives above does not own the flat but I will have to try and find out who owns it and get them to sort out the problem! Just what I do not need as I am off to house and dog sit from Thursday till a week on Sunday!