They bury themselves like that a lot, it's normal. Your temps are a little on the low side though; for adults it's good to have ambient temp 80-85 day and 75-80 night, and for babies don't let it get over 81-82. Mine will start burrowing completely if her temps go down to the 70 degree mark.
Clean like normal. If you get him out and give him a luke-warm soak in another dish while you clean the tank he'll probably enjoy it