How to understand yin and yang

I’m not entirely sure where you are reading about the idea of yin excess?
Regardless, there are many theories and views and one is certainly that there is no such thing as yin excess - dampness, yes, but yin in excess is not a possibility. A common, but far from the only, diagnoses involving psoriasis/eczema is yin deficiency, however.

On our eczema treatment page, for example, you see the following patterns as possibilities (not the only ones mind you):

heat in the blood, kd yang deficiency, kd yin deficiency, liver wind, liver yin deficiency, liver and gallbladder damp heat, sp yang deficiency.

Those should get you going in the right direction towards formulating a treatment plan/concept.