The dust upon the mirror, that Rumi is speaking about, is thought taken from the mental realm.
The thinking mind IS the veil that separates you from your true Self as Spirit. Not thought but thinking. One needs thought to be in the world, however this thought should be in a state of being - not thinking if we are to give life to the truth rather than an illusion.
The mind veils the true Self by using the true Self's Eye/I for creating thought, rather than keeping it empty so Awareness can be delivered intuitively for the mind to process.
The I am-ness of our being can be used in two ways: firstly, the right way in which the being (meditative) mind, provides a passage for Spirit to flow into the world via intuition or the wrong way, whereupon our I am-ness (That should be filled with the Holy Ghost aka Intuition) creates and takes thought instead.
The conscious mind is a wave that rolls out of the subconscious awareness which is the door to the superconscious - Soul/Spirit. If it rolls out of the sea it is no longer in its temple and available for tuition from one's true Self.
While the mind is present in thought and absent in his temple of awareness, the mind is not free to marinate within its own being or awareness therefore unable to realize deeper dimensions of its own existence. Unable to see and know its own Soul, its own Spirit, it mistakes itself for being a mind and body only.
Thinking is the slayer of real. Being is redeemer.
Therefore, be mentally still, quiescent, and intuitively receptive to the truth of your Being, keeping the mind focused in and as Present Moment Awareness that is both the subconscious and Soul, while in an intuitively receptive state. Be mentally still and now that I AM.
Learn to see with your Inner I am-ness rather than the eyes of flesh.
I once was lost, but now am found T’was blind but now I see ~ John 9:6–25