Get some super glue and glue it down that'll teach it to move around! Lol jk I would adjust my flow in a new direction if there was a desired location I wanted it to be in hopes it would stay there it worked for me a few times.
Yeah, my first anemone was a a quarter sized and it moved a lot until it died when it got lodged under my rock. Had to take it out and clean the rock. However, on my second anemone I made a crevice on where I wanted it at. It's been almost 6 months and it hasn't moved. I got lucky this time, really enjoying having my anemone in the front.