You are not eligible for an HSA after you have enrolled in Medicare. If you had an HSA before you enrolled in Medicare, you can keep it. However, you cannot continue to make contributions to an HSA after you enroll in Medicare.
You will report the distributions from the HSA on Form 8889 -Part II under distributions and report this as used for medical expenses and hence....the distirbution will not be taxable.
Let me know if you have any question.
Please note: This advice is provided with the understanding that all the relevant facts have been provided by you. Any change in facts might affect the advice given and hence may not be relied on in such cases. Nothing contained in this reply was intended or written to be used, can be used by any taxpayer, or may be relied upon or used by any taxpayer for the purposes of avoiding penalties that may be imposed on the taxpayer under the Internal Revenue Code of 1986, as amended.