If failed to appear on prostitution charge is that a felony?

No, not unless the charge you had was a felony charge. However, if you FTA'd there will be a warrant out for your arrest, and you're in a high-exposure (no pun intended) occupation. Most street cops know who the 'regulars' are and where to find them - so you're an easy target. You'd be better off turning yourself in and taking care of it.