I've had peppermint shrimp eat smaller rock-flowers, though generally only if they were exceptionally pretty, such as the yellow ones with deep green centers and flaming red tentacles. The RFAs with only one or two colors, they've left alone.
Not the same exactly, but when I added a Bubble Tip Anemone to my tank the Peppermint immediately went after it. The anemone was very stressed from shipping and the Peppermint probably could tell it was weak. I kept the shrimp away for a few hours and after the anemone filled in and secured himself, the Peppermint never seemed to bother him again.