Wash the straps, dry 'em, use it. Same goes for the little paddy bits which otherwise 'salt up' big time (like
@speccy1 's straps). Please to say today was so warm that for the first time this year, this was a (welcome) issue (and now the 'paddy bits' are washed and drying, and the helmet sans pads, is hung up to dry). Weather forecast's good all week, though bit windy on some days.
Helmets (well the 'chosen one') should always be kept with other important items (eg gloves, specs, emergency pack/bag, tube) in a 'grab bag' in case there's a chance of a ride at short notice. But if they're left damp (ie the straps, after a ride) then stored in a (?damp?) shed, mildrew may thrive. Even if dried, the salt remaining (from dried sweat) will be hygroscopic, with likely the same end result (in a damp environment).