Is this contamination?

If it's white, it's healthy:

1 4

The white stuff is the 'roots' of your mushrooms.

If it's white with yellow droplets, that's also healthy:

1 3

The yellow droplets are waste products of the mushroom's roots.

If it's any other colour, that's contamination:

1 3

Send a photo of your contaminated grow bag to hello [at] getmentis.com. We'll replace it for free.