Goggle is telling me that a 'h2o mop' is a steam cleaning device (from the USA), and if so you'd risk ruining your plastic fermentors for no real benefit.
A good soak in hot water and sodium percarbonate then a light wash with soft cloth is all you need to do to clean them well, why bother with anything else and why use something that would damage them?
Having said that some people do use their plastic fermentor's as a 'bucked of death' to boil water, wort and stuff in so, maybe you could get away with it, I just don't see the point.