Its probably moving around because its not happy. I have even seen them puff up like a balloon and float away. Very easy to see how if he was floating around he would get sucked into a PH.
To me that is the hardest part of keeping an BTA, just getting it to settle in and stay. Once it finds a spot it likes, it will attach and stay there for years. I have had them detach move around then entire tank and then come right back to where they were to sit for another year or two.
fwiw,when I introduce them, I usually put them at the base of my rocks, and let them climb up to wherever they like the flow and light levels.
edit: i see you did put him on the sand. possibly move him over to the other side of the tan, and see if he likes those living conditions any better