Thanks Jan! Yeah, I aleady knew this about natural, undyed Malabrigo Merino! I've used it in the past, and got the same filthy "bath water"! I read it's important to remove this dirt because it can grind on your yarn fibers, and it makes the yarn pill even quicker than normal. It is hard on the fibers, too, is what I read.
The normal kettle-dye process for colored Malabrigo removes a lot of that dirt. But the natural/undyed cream doesn't get that second plunge!
I've read some Ravelry notes that even their colored Malabrigo has hidden dirt that came out in the "bath"...along with dye of course. But they could still see that there was 'dirt' too!
So maybe Malabrigo Merino yarn is especially dirty. Maybe the Malabrigo folks don't want to unnessarily 'handle' the yarn more than absolutely necessary. It is very delicate, and it's a single ply, too. If you squeeze it too vigorously in the bath water, it will tend to start to felt! You have to be very very gentle with it.