It's not written anywhere that I can see, but you need to click on them to get them moving again.
That does work, but I'm pretty sure it's not working as intended. The description of the attack states that it freezes balls for 15 seconds, however they don't unfreeze after that time period. If you're supposed to click to unfreeze after the 15 seconds, that should be mentioned somewhere, and clicking to unfreeze works even if 15 seconds haven't passed. The way I see it, the attack is intended to work in one of three ways:
1. Balls only unfreeze after 15 seconds and clicking does nothing (how it seems to be stated to work), in which case the unfreezing needs to be fixed and the clicking needs to be disabled
2. Balls only unfreeze from getting clicked (how it currently works), in which case the attack description needs to be changed
3. Balls automatically unfreeze after 15 seconds but "secretly" can be unfrozen immediately by clicking (how it's probably intended to work), in which case the auto unfreeze needs to be fixed