In psychosis, what is the difference between tic and mannerism?

Not in psychosis. Neither is typical for psychosis though of course may incidentally co-occur. A tic is a short tightening and release of a small group of muscles. If you force yourself not to, tension will grow increasingly uncomfortable, squeezing one's eyes tightly shut, then opening is a common tic. Mannerisms are more complex gestures. They are just habitual. Multiple persistent tics comprise tourettesyndrom.