Under almost all circumstances you should never try moving an rbta. It is very hard to dislodge a nem's foot from rock without injury. Nems will hide from light for a couple reasons...depending on the tank's parameters. If your tank is well established with stable/pristine water conditions, it may seek a darker/more sheltered location for it to split. Nems will split when they are healthy and deem it a good time to "reproduce". Nems will also split when they are very stressed. So, if you haven't tested your water in a while, now would be a great time to do so. It may give you better insight into why your RBTA has decided to hide a little. Your RBTA does, however, look to be healthy. Nice and deep in color. Look for an elongation of his foot. If he is going to split, he will try getting a good grip in a wide stance and literally pull himself apart.