Once it shows up, S. marcescens is hard to get rid off (it's an airborne bacteria) so do increase the humidifier cleaning schedule. Would check with manufacturer to see if could run a standard 1 cup bleach/gal of water solution through it and soak all non electric parts for 15-20 minutes in bleach solution. Then rinse well and soak on a 10X strong dechlorinator solution. Should be nice and clean after that.

Since it's airborne, if you have other sites in house with it (check toilets and showers) there is possibility of relapse, so will need to clean other areas and the humidifier more frequently.

S. marcescens is pathogenic to some humans; but got no references to it affecting frogs. Imagine it's the same way and could affect unhealthy or compromised immunity individual ones. Good luck !