I just got a BTA, and they move around a lot more than I anticipated. I initially placed it on a rock, it let go and plopped onto the sand, so I held it on a little longer, and it was able to hold on, but then it immediately crawled around the rock to the backside where I could barely see it. Every morning is an exciting adventure where I get to hunt around the tank to see where he's gotten today.
Yesterday he was towards the topmost rock, nearly where I put him in the first place.
But for the first few days, to almost a week it really felt like he was finding a nice secure crevice where it could be left alone while it adjusted to the new tank environment.
But what should you do? I personally don't think it looks happy on the sand like that, so personally I would pick it up and hold it against a rock as close to right below your light and at the top of the tank and try to get him to hold on. It could take longer than you think. I think I had to hold my dude for maybe five or ten minutes before he was on there good. Then be prepared for him to move around until he's happy.