Read the label very carefully. It is most likely a bag of Erythritol.

I haven't used it as a sweetener per se but it's used in a bunch of products I like. E.g. energy drinks or protein. Those don't list erythritol. You're right that it's tricky (impossible?) to find on its own. I suppose it needs to hang onto something to get it into crystallized form?

I find monk fruit to be utterly unpalatable.