What is that brown film on the cup when you make tea?

Question

Is it bad?

Answer 1

no it is just what happens to milk when it is heated, you get it on custard and dishes with cream also, it is unsightly but completely harmless.

Answer 2

Actually all tea contains dust which is essential to get a rounded and proper taste from the brew.  Some of this fine dust rests on the surface and can cause a slight film on black tea or interact with the cream in milk to do the same.

It is perfectly OK and the tea would not be so good if you found a way to prevent it happening!