Mazu generally taught that mind is buddha. Then there was neither mind, nor buddha. Dazhu Huihai was quite satisfied with the first teaching, others preferred the latter. But without understanding the meaning it's pointless to debate the words. The meaning obviously is to see mind's true nature, what else is there? Then look at the different teaching methods used.
A monk asked, "Master, Why do you say that Mind is Buddha?" Mazu said, "To stop babies from crying." The monk said, "What do you say when they stop crying?" Mazu said, "Neither Mind, nor Buddha." The monk asked, "Without using either of these statements, how would you instruct someone ?" Mazu said, "I would say to him that it's not a thing." The monk asked, "If suddenly you met someone who was in the midst of it, then what?" Mazu said, "I would teach them to realize the great Way."
"If the Buddha-Nature is seen, there will be no seeing of a nature in any thing."
(Nirvana Sutra, T12n374p521b3; tr Yamamoto)
"Neither cultivation nor seated meditation — this is the pure Chan of Tathagata."
(Mazu Daoyi, X1321p3b23; tr. Jinhua Jia)
“Don’t rashly seek the true Buddha;
True Buddha can’t be found.
Does marvelous nature and spirit
Need tempering or refinement?
Mind is this mind carefree;
This face, the face at birth."
(Nanyue Mingzan: Enjoying the Way, tr. Jeff Shore; T2076p461b24-26)