Not enough information....
Is it new or used ?
What sort of a PB is it?
Have you tried to clean it and with what?
Are the immediate questions that spring to mind.
The accuracy of responses is usually directly proportional to the quantity and quality of information behind the question.
It's a brand new pressure barrel from wilko. Cleaned and sterilised properly. It's a golden ale. Followed the instructions correctly and the beer actually tastes pretty good, although a bit cloudy.
@Paulshezza
Your beer should clear after it's carbed up, which can take anything from 5 to 15 days or even longer depending on how much priming sugar was used, how much yeast was in there to start with, and the carbing temperature. Plus some yeasts take longer than others to drop. Also its always a good idea to leave your beer about a month or so from packaging before you try it, which allows for not only carbing up but also some conditioning too. By then it should be clear, but may or may not be quite ready to drink or at its best
Can't help with the 'coating' I'm afraid, but if it was OK before you filled it with new beer it is most likely to be yeast as you have said.