In the Shen-xiu’s statement it is essentially stated that our mind is originally pure, but it is always being defiled by our negative experiences in life.
In this statement, the mind is effectively a passive thing, which could gather dust, so we must keep it cleaned from impurities.
If we wipe away those impurities we may be able to eventually get to enlightenment.
But Hui-neng s statement suggests that there is no duality between one’s original mind and one’s actions, or between purity and defilement. In fact, there was no original mind.
So The master perceived that Shen-xiu’s statement understood the mind as having substance, while Hui-neng’s statement indicated that it did not.
In Zen teaching, the original mind was understood to be clean and to remain pure as well. There is no need to be concerned with trivial matters like keeping it clean—it is always clean, just as the Buddha-nature is always in us, ready to be awakened.