A Cryptoplasm is a Mirran agent who infiltrates Phyrexian compounds by magically changing its own shape. In the art, you can see that the Phyrexian-looking figure in the foreground has taken the shape of the fallen Phyrexian in the background—the survivor is the Mirran spy, the Cryptoplasm.

"If left in the enemy's shape too long, it might be lost to them." -Vy Covalt, Neurok agent