What is the cause of hyperpigmentation. is it related to high levels of acth?

Multiple reasons: Hyperpigmentation may be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris. In cushing's disease, there is excessive adrenocorticotropic hormone (acth) production, which leads to hyperpigmentation because msh (melanocyte stimulating hormone) production is a byproduct of acth synthesis from proopiomelanocortin (pomc).

Cacan be: It can be because stimulates more pigment. See your dermatologist for evaluation and to discuss treatment options.