How did the Middle Easterners and Chinese react to seeing each other at the Battle of Talas River? Had Middle Eastern Muslims or the Chinese ever made direct contact with each other on such a large scale before?

Had Middle Eastern Muslims or the Chinese ever made direct contact with each other on such a large scale before?

Depending on what you mean by "large scale":

Trade between China and the Middle East existed long before 751CE. China had sent people west and Persians and Arabs had gone east before, with records* of contact during the Han period (206 BC – 220 AD) in the form of Chinese expeditions to the Indus Valley, and Arab envoys into the Tang in the late 600s.

They certainly knew of each other, in person, by the time of the Battle of Talas.

I can't speak to reactions, the first part of your question. /u/kevink123 or /u/lukeweiss will surely be able to answer this better. Name dropping to get their attention.

* China's Muslim Hui Community: Migration, Settlement and Sects, Michael Dillon, 1999, referring to the Shiji 史記 and Hou Hanshu 後漢書