I agree. My plate coral is HUGE in the afternoon and in the morning it is really small. Yours looks good to me. I have a Long tenticle so it would look a bit different than yours.
However like lando said, positioning is the key with these. My long tenticle was moving because he didnt like the flow where I had him in my old tank. He was actually crawling up this rock that I put in the way to keep him from my brain. Every couple of days I would have to slide him back over to his spot so that he didnt get to close to the brain. Anyway if they arent happy where they are at you'll know it. If he's sucked in all the time then he is stressed out. But if its all puffy like it is now, its doing just fine.