I bought it from Rona, it doesn't have it at first, but it appears afterwards. Perlite? The soil is for growing grasses.Is this something you bought from a garden store? It's probably perlite. It's supposed to help keep the moisture in the soil. Try to find some without perlite.
Could be fungus starting. Or possibly some sort of mineral left behind after some water evaporates.
This is why I don't like "soil" or "top soil" as a tortoise substrate. "Soil" can be made of any composted material and you have no way of knowing what it is. Many potential soil components are going to support fungal and bacterial growth. Fuel for microorganisms, if you will.