Why is there white clouds in my minitor?

It sounds like you have condensation collecting on the inside surface of the cover glass. Do you live in a damp cold climate? Moving in and out of cold and heated spaces can cause this.

A second possibility is running a humidifier with tap water. While this sounds odd, what happens here is the humidifier atomizes the water and in the process the minerals in the water create a very fine dust. This dust tends to collect on a statically charged surface like your display panel and cover glass.

I had this problem in my iMac at home after cleaning the house fully of the fine dust with a HEPA filtered vacuum of this fine dust. I now only use distilled water in the winter for humidification as my dogs need it.

In either case you'll need to take the cover glass off and carefully clean it and the LCD. Only use reagent quality Isopropyl or grain alcohol as anything else with streak.

Follow this IFIXIT guide: MacBook Pro 15" Unibody Late 2011 Display Replacement on getting the display assembly off. Then follow this IFIXIT guide: MacBook Pro 15" Unibody Late 2008 and Early 2009 Front Display Glass Replacement picking it up from Step 14. Make sure you have a variable temp heat gun as you can over cook the plastic & rubber parts.

I should point out this is not an easy fix. You may want to consider just replacing the full assembly with a new unit Vs the repair or if you are near an Apple Store see how much it will cost for them to do it. Often I find Apple can do things cheaper than I can.