What is the ruling on the presence of "naku" (dried residue) on one of the parts of ablution, along with the diligent effort to remove it and the awareness of a small amount of it or its trace remaining after prayer?

That which has a substance and prevents water from reaching the skin must be removed for ablution. That which has no substance, like the color of henna, does not prevent the validity of ablution. One who is unable to remove that which has a substance has a valid purification. There is no difference between a small or large amount of that which has a substance. Some scholars, like Ibn Taymiyyah, have overlooked a small amount that prevents water from reaching the skin.