prop that bad boy....er, girl up, and put a cfl or t5 close to it till it thickens up, thats just looks like it tried to grow too fast to reach your light to me
i've seen a couple of people have this problem, the light is too strong, even dialed down, if you have it close enough that the plant won't stretch for it, it will burn it, if you put it far enough away not to burn it, it stretches to reach it. don't use hid to start plants, get a couple of cfls or t5s till they get established, like the 3rd or 4th set of leaves anyway.