i would say that to put some kind compound on there in an attempt to fix things would be treating a symptom not the cause (if it is indeed a steel frame and an alloy post). so i'd look for the cause of the problem.

OTOH i don't think that stuff will damage the frame. and it would be helpful, in guessing, if we knew whether or not the post was ever secure, and if so, what has been changed.

i'm pretty sure the problem will turn out to be something simple.