How unusual  is this [mouldy mushrooms]?

The mushroom caps you buy and eat are the fruiting bodies of the fungus. The actual fungal body is called the myceclia and is a threadlike mass, often white in color that infiltrates the source of nutrients. It isn't unusual to see mycelia develop from mushroom caps in the refrigerator.  

I avoid that slime mess by emptying the carton of mushrooms into a paper bag and putting it in the fridge. The mushrooms will slowly dry out, but they won't spoil. If they get too dry, make duxelles.

That is fairly common as mushrooms are fairly damp to start with and will easily mildew if they are not stored properly.

Fresh button mushrooms are white, although they can get darker in color. This does Not mean they are spoiled. But why would you even buy mushrooms if you are not going to use them in a day or two? 

Not unusual in my fridge. I have resorted to the frozen mushrooms from Whole Foods. I know I'm making the gourmets on here cringe, but it saves a lot of waste.