Is your tank quite new? It looks quite new .
check your parameters and post so we can see if anything obvious there.
generally, anemones are crazy creatures and they go through all sorts of contortions when they are settling into a new tank even when everything is perfect. It’s normal for them to hide in a cave, behind the rocks even in complete darkness, or deflate and re inflate ( equalising the water ) the best thing you can do is leave it alone ( assuming your tank is old enough and parameters are reasonable and stable). If not you might consider housing it in a more suitable tank until your tank is ready.
The anemone has good healthy colour, it’s attached and its mouth isn’t gaping, all good signs.