Assalaamu alaykum. I have two queries:
1. My prayer gown has men's shirt-like sleeves with buttons to fasten at the wrist and a slight opening above the buttons. The sleeves fully cover my arms; however, during some positions of the prayer, like when I raise my arms, a bit of the skin of my forearms can be seen through that opening. Does this amount to my ‘Awrah (what should be covered of the body) being uncovered? Could it be that some of my past prayers were invalidated due to this reason? Please clarify.
2. I used henna on my hair mixed with tea water, coffee, and yoghurt as a hair pack during my periods last week. After washing it, this left a greasy residue on the comb. I shampooed my hair a couple of days before my Ghusl (ritual bath) and did not repeat the shampooing on the day of Ghusl. As I combed my hair after Ghusl, I could see the same greasy residue on my comb; does this mean that my Ghusl was incorrect as the 'dirt' had not been washed off my hair? Do I need to repeat my prayers and fasting which were done with this Ghusl?
I look forward to your response on both these queries. Thank you.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Regarding your first question, you should know that covering the ‘Awrah is one of the conditions for the validity of the prayer. However, if a slight part of the ‘Awrah is revealed out of ignorance of the relevant ruling or by mistake, this does not affect the validity of the prayer, even if it remained uncovered for a long period of time.

Hence, if you suspect that a part of your ‘Awrah was revealed during the prayer, then you should consider that it was not revealed, because doubting the validity of an act of worship after one finishes performing it does not affect its validity. Please, refer to fatwa 119112.

Even if we assume that a part of your arm was revealed during the prayer, this was a slight part, so it is excused, and you did not reveal it intentionally; therefore, your prayer is not invalidated by it. If you become certain that a part of your arm is revealed during the prayer, you should hasten to cover it immediately, and you are not required to make up for any of those past prayers.

Regarding your second question, please submit it separately so that we could answer it, following the website's policy (of submitting one question at a time).

Allah knows best.