This thing is calcium sulfate.

Nan tofu, also known as "gypsum tofu", uses gypsum as a coagulant. Gypsum is calcium sulfate, and its solubility in water is not high. The "network" nodes formed are sparser than that of northern tofu, and the resulting tofu is more delicate.

Northern tofu is cooked using brine (liquid obtained by adding salt brine and water), and is also called brine tofu or firm tofu. The cations in brine are mainly calcium and magnesium, and the anions are mainly chloride ions and sulfate. Both calcium and magnesium can form stronger links between protein molecules, and their higher solubility can make the nodes of the "network" denser, making the tofu harder and rougher.