It was in an Eisbiber's nature to run away or hide when it was in the presence of a Grimm. Nick was easily frustrated when talking to Bud or one of the others because they would fidget constantly. It was natural instinct they were fighting and Nick was sympathetic to a point. But the more he spent time with Wesen, the more they should realize that he was only a threat if they broke the law. Human law.
But still, none of them seemed to trust him as every Wesen always had their guard up when talking with him. Even Monroe and Rosalee.