Odysseus does not warn his crew about Scylla because he thinks they would

No because he know that he would

Man, I love Greek mythology, I know basically everything from it ...

Odysseus didn't want to warn his crew about the cliff monster known as Scylla (long story short [Heh, get it?], lots of heads on long necks, heads pluck dudes off their ships and eat them) because he did not want to send them into a panic, which would endanger both all of the lives on the ship, and probably the ship as well.