The impression I got is that whatever Renarin's light did to make the thunderclast go may be similar to what he did to Moash. In OB, Glys basically said "It will fear you", in relation to making the thunderclast go with the light. Again, we don't know exactly what that light did, but it was a beacon of light. Now, when he shows up after Kaladin, there's a burst of light, a presumably better version of Moash is seen, while the real Moash first shies away from the light, fights the empty air, and then rushes back up the tunnel.
Maybe it shows the target something they don't want to face? I don't know what could possibly scare a thunderclast, but Glys explicitly commented on fear there on the thunderclast scene. And Moash seemed pretty shaken now. Possibly afraid of that brief thing he saw. Would be pretty interesting if, one way or the other, that light made people face things (say, truths) they wouldn't want to face.