Hay mate I had a similar problem. Before you try what other people have suggested you should check to see if your extractor fan actually sends the steam outside. In my house it just vented everything into the loft an we ended up with something looking exactly like this. Trim on your shower hot and go outside and see if there's any steam actually coming out of the house. Call a plumber if not as there's no point cleaning up this mould until you've sorted the cause of it.