Mine are the same way, all three like to have there foots underneth something and then poke there heads out to collect food/light. I guess they do this so that in case there is any threat, they can retract and be hidden under something.
The BTA's will comeout once they do not get enough food/light in the caves.
You can point a small powerhead at them if you cannot move them and they will start moving again until they find another place they like. Just make sure theres NOT so much flow as to tear its flesh.