What is the name for 'curdling' of coffee by milk?

The one word you are looking for is flocculation.

Here is a quasi-scientic--but helpful--explanation about what happens when you add milk to your iced coffee:

Curdling Chemical Reaction

Fresh milk is an example of a colloid, consisting of fat and protein particles floating in a water-based solution. The colloidal suspension scatters light, causing milk to appear white. The protein molecules, mainly casein, repel each other so they naturally distribute evenly through the liquid. Milk is slightly acidic. When the pH is lowered even more by the addition of another acidic ingredient [such as coffee], the protein molecules stop repelling each other. This allows them to stick together or coagulate into the clumps known as curds. The watery liquid that remains is called whey.

The jist is that when two acidic liquids combine, there is curdling, or flocculation.