Transient, fleeting or ephemeral.
Elusive or difficult to retain.
Fleeing or running away; escaping.
A person who flees or escapes and travels secretly from place to place, and sometimes using disguises and aliases to conceal his/her identity, as to avoid law authorities in order to avoid an arrest or prosecution; or to avoid some other unwanted situation.
Ephemeral or relatively short-lived.
Lasting for only a moment.
Happening at every moment; perpetual.