mold is quite common on rhizomes. it's almost to be expected - they're kept damp, dark and with little air circulation. that's a recipe for mold. same thing happened to mine in the fridge, freaked me out at first.
wipe 'em down, and do a wash and very light scrub before planting.
do a search for "rhizome mold" (using the search box at the top right of the page, inside the beer mug) for a bleach dilution you can dunk them in if you wanna go hardcore.