Ugh... I feel your pain. Bust out the gravel vac and be resigned to it taking a few passes. On the plus side, Purigen's a lot lighter than gravel, so it does vacuum up readily. Plus it's safe enough to leave in for a few weeks/months... just let the flow sweep it into a few places and clean them out from time to time.
This happened on my nano tank 8 months ago and I'm still finding beads of it from time to time. other than the annoyance, no big deal.