If it stuck itself there I would say it's pretty happy.
If it starts retracting or lifting up and moving around I would keep a close eye on it just to ensure it foots again.
Im struggling myself as I am sitting here wondering how anyone on here can possibly discern the emotional state of this invertebrate from a single photographic image?
I suppose happiness is a possibility but it might be simply content, or completely bewildered wondering how it got into your reef tank?
Sorry, I’m just waking up and considering the possibilities of an anemones self awareness has me mentally overwhelmed by the question.