Dandruff on your scalp, redness and flaking on your face and a thick big toe nail, often indicate seborrheic dermatitis.
New cell growth starts at the bottom of your skin, then another new cell grows under it, gradually pushing the cells above towards the surface. The outer layers flake off as dander and dandruff. The ring that remains on the tub after you bathe is most likely old skin. It usually takes 28 days for a skin cell to pass from the bottom to the top and be sloughed off as dandruff.
The usual treatment is to remove the scales by shampooing the face and scalp as often as tolerable. Many dermatologists also prescribe antibiotics, like ampicillin or doxycycline, because surface bacteria increase the rate that the skin turns over.
A non-prescription method of removing scales is to massage a small amount of the Dermal Therapy Extra Strength Body Lotion into the scalp after a bath or shower. This will reduce the amount of dry scaly skin and improve the appearance of the hair. Be careful to only apply a small amount to the scalp or the hair will become greasy.
American Academy of Dermatology – Seborrheic Dermatitits https://www.aad.org/public/diseases/scaly-skin/seborrheic-dermatitis
American Family Physician – Treatment of Seborrheic Dermatitis http://www.aafp.org/afp/2000/0501/p2703.html
National Rosecea Society – Seborrheic Dermatitis https://www.rosacea.org/patients/seborrhea.php
British Association of Dermatologist – Seborrheic Dermatitis http://www.bad.org.uk/shared/get-file.ashx?id=180&itemtype=document