Not being familiar with your medical condition, I looked it up. You need frequent blood-letting to keep down the amount of iron in your bloodstream. This procedure, I imagine, is not dissimilar from the basic blood test that many people have to have.
You'll find English (or English speaking) medical centres in the Adeje area listed on my website who may be able to help you. Whether any treatment received at one of these would be covered by your medical insurance is something you'd have to contact the insurance company about.
Regarding the public health system, you would be only eligible for free treatment if you were registered with and making payments to Spanish social security.