Answer: hemozoin pigment
This pigment is formed as the infecting Plasmodium species feeds on the host's hemoglobin in the red blood cells. This young girl had previously been diagnosed with a heavy Plasmodium falciparum infection (3.5% parasitemia). Here are some images of her previous thick and thin blood films.
Fortunately, she made a full recovery. Thank you again to Emily F. for these beautiful photos.