First, it's the hand that grabs the target, not you (the PC). Thus, the PC can still move around while the hand is grabbing something -- just be sure to stay within range.
Second, the power seems relatively clear about how you can use it after the initial attack. When you sustain minor, the grabbed target takes 1d8+Int damage.
After the sustain, as a standard, you can attack "another target" -- the intent seems to be: it can't be the same target the hand is currently grabbing.
What's unclear is whether the intent is that you can move the hand around after it intially appears. The way the power is written, I'd say no, you cannot. That being the case, the only time you'd get to attack "another target" would be if a monster ended up adjacent to the hand.
If this power were being created today, it would be better written. It might actually make more sense as a Summon!