its a murray, white line along the mouth under the eye is a give away, kreffts are simular as hatchies but the line is above the eye. no way on earth is this a long neck.
hatchies are best of kept in a tank as you can maintain a better temp and keep an eye on them better, also in a pond cats and birds may attack him as he is really small. water temp 24-26 degrees is best, make sure you have a uvb light withing 30cm of his basking spot use a turtle dock. also get some turtle calcium and water conditioner blocks for the water, feed him a variety of food e.g. turtle dinner frozen cubes, chopped up shelled raw prawns, raw fish, live guppies or feeder fish, blood worms, turtle sticks, live crickets and woodies, duck weed.
if you have anymore questions i'm happy to help, i keep and breed murrays and a few other turtles.
to sex it i would wait till its older to be 100% sure, males have longer tails than females and have more of a v shape than females.