As the snow is melting I'm getting a mould on my grass in various spots. I noticed last year that some of the grass turn to white powder.

That is snow mould and it is very common. The first thing I want you to do is... nothing. I want you to wait until the grass has dried to the extent that you can walk on it without leaving an impression from your shoe on the grass. Once the lawn has dried to that extent all you can do is rake it gently with a broad rake and get the grass blades to stand on end. The air will circulate through the crowns of the grass plants and this will discourage the snow mould. It is important to fertilize the lawn with CIL Golfgreen after you have gently raked the lawn.