Tranquil Frillback's triggered ability goes on the stack with no targets and no modes chosen. As it resolves, you may pay {G} up to three times. That is, you may pay {G}, {G}{G}, {G}{G}{G}, or choose not to pay. If you chose to pay, the second "reflexive" triggered ability will trigger. You'll choose the modes and targets, if any, for that second ability at that time.