I was looking up what to do with those Mandala yarns, and I saw where they said you can deconstruct the whole cake into its separate colors! Seems like a waste! But, on the other hand, if you think of it as buying a “set” of coordinating yarn colors, then separating them might be easier (cheaper?) than buying several separate skeins of yarn. At least you would know they look good together. I don’t plan to deconstruct any of mine.
If the musty smell came from the mat, then I would not use that mat again.
As for washing crochet, I only have afghans. But they stand up to machine washing very well. And drying. Maybe you could wash it on delicate cycle or by hand, and then lay it flat to dry, but not on the mat? Seems like the mat doesn’t allow enough air flow. You could also consider machine drying it for just 10-15 minutes to get a good portion of the water out, and then lay it to dry.
That’s what I do with my store bought sweaters that require laying flat. And I lay them on this rack I have that’s like a lot of plastic clothes lines in a row. So technically not 100% flat. But it works.