This has happened to me too, however, it didn't damage my new mutant with 2 damage per turn. It was just a symbol, and did nothing more.
I had the same experience. In the second picture, the two on the left were mutated Malignant Cells. The symbol had no effect. The one on the right was a creature that was infected and was dying.
(
http://www.screenshotdumpster.com/view/m7Gbd48995/afla1)
(
http://www.screenshotdumpster.com/view/m7Gbd48994/afla2)
I didn't try devouring one to see if another Cell popped up, but I did kill one in a failed mutation, and it did not create another Cell.
[
edit: Okay, I just played the same AI5 again, and I see what you're saying, Avenger. If you devour a creature
still marked with Aflatoxin, it will create a Malignant Cell (and a Skeleton if any boneyards/graveyards are out). Once it is actually a Cell it lives and dies like a normal creature. One thing I have not checked is devouring a creature with the darker symbol that appears after mutating a Malignant Cell, but I suspect it will have no effect, since it doesn't act like Aflatoxin.]