I'm fairly certain Universal only has one high-reach lift that could get that high, which is used predominantly by the techs for light installs. I'm not sure if that reach could get THAT high, nor across far enough to power wash the whole roof.
Whenever they'd do it, the cleaners would more likely harness in and walk down. I'll give you they can do it in a night, but I'm not sure who would do it. Building maintenance and techs are certified to harness, but I don't know if I'd want them doing; custodial (who have the power washers) aren't. Besides, all of them already have overnight repairs to be concerned about.
Best option is to contract out building cleaners. That way, if the roof is damaged, it's on them.
Either way, it's mildew in a "non-guest" area. I hate to tell bio, if he thinks that's a "problem," he'd lose his mind if he saw some of the other shenanigans backstage.