Grey mushroom cream sauce - why so dark?
You don't say how you prepare the sauce, and Alfredo does not contain mushrooms (or flour, or onions). But if you are simply pureeing button mushrooms, then no, there is no way to make it any other color. The mushrooms have black gills, and they will color the whole white sauce. 

In theory, you could try removing the gills from button mushrooms before cooking a sauce. In practice, you will probably have enough pieces remaining to give the sauce a light greyish tint. 

You can use a different type of mushroom, one without black gills. Porcini should work. Of course, you'll end up with a different taste, and porcini are not a staple in every supermarket. But if you insist on a white sauce, this is the way to go. 

Another option is to just eat it as it is. Humans need some time to get accustomed to visual stimuli which are not expected in a given context. But a sauce made with mushrooms, onions and cream is super tasty. After you have eaten it a few times, you can learn to associate it with "good", not "unappetizing". But I guess the success of this method will vary between people.