This is an anti-magnetic coating called 'Zimmerit'. Developed in 1942 and introduced to AFV-production lines in late 1943, it stopped being applied to vehicles from September 1944 as a cost-saving measure.

The development of the coating emerged somewhat isolated from the development and mass-production of the hafthohlladung. This was a cross between an anti-tank mine and an anti-tank grenade, to be used by infantry formations as magnetic adherent anti-tank weapons in close combat with enemy armour. The device was essentially a magnetic shaped charge that would stick to flat metal surfaces. Its combat record was mixed; though it was highly effective when applied, it's close combat nature rarely saw it applied.

Zimmerit-coating was developed as a countermeasure against magnetic mines of similar produce and capacity. From a military philosophy and technology perspective this is a sound move, but in practice turned out to be an expensive misprioritisation; namely, no other military opponent of Germany at that time used magnetic mines or such devices in front line combat. The fact that the zimmerit coating was also difficult and expensive to make led to its phasing out in the late summer of 1944 as it became clear that the combination of AFV-losses suffered in France and the expensiveness of the process of the synthesising + application of zimmerit coating further hampered the already reeling military production lines.

Chris Bishop's Encyclopedia of Tanks and David Miller's The Great Book of Tanks were of much use in writing this reply.