I suspect the problem may not be the moisture itself but what it has done to the PbI2 surface. Quite likely you now have hydroxide (Pb(OH)2 or PbOHI) and maybe some lead carbonate.

If that's the case, you should probably treat your powder with an HI solution. I don't know what concentration is best but I think that highly concentrated HI may cause significant PbI2 loss. Most residual moisture can then be removed (before it damages the surface again) with a little dry DMSO.