بسم الله الرحمن الرحيم
(Fatwa: 135/139/SD=3/1438)
On enquiring about the henna mentioned in the question, we came to know that it gives colour like normal henna after applying, but after some days the colour becomes darker and gradually starts shedding off. It is not the layer of the henna rather such brand of henna contains chemical which causes the skin to shed off along with shedding off the colour. However, wudhu and ghusl are valid after applying such henna. It is not the layer of the henna which could stop the water to reach the skin.
Yes, if it is the henna with layer then wudhu and ghusl shall not be valid without removing it and if with much effort the layer does not remove then wudhu and ghusl shall also be valid with it due to necessity. But it is necessary to make efforts to remove such henna in wudhu and ghusl.
وفي الجامع الصغير: سئل أبوالقاسم عن وافر الظفر الذي يبقى في أظفاره الدرن، أو الذي يعمل عمل الطين أو المرأة التي صبغت إصبعها بالحناء أو الصرام أو الصباغ قال: كل ذلك سواء يجزيهم وضوءهم إذ لا يستطاع الامتناع عنه إلا بحرج، والفتوى على الجواز . (الفتاوى الهندية 1/4)
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband