Peace be upon you. According to Grand Ayatollah Sayyid Sistani (may Allah protect him), it is not necessary to remove dead skin or wash underneath it. However, if the dead skin is considered an external substance, it must be removed. As for Grand Ayatollah Sayyid Khamenei (may Allah protect him), he states that for skin conditions like scabs, it is sufficient to wash the surface, and there is no need to ensure water reaches underneath the skin. Even if part of the dead skin is removed, washing the surface of the remaining part is sufficient, and it is not required to remove all of it. May you be successful in all good.