Seems like "evaporate" is the wrong term for something that's already a gas cloud. "Dissipate", maybe?

Could a gas cloud only be dangerous (to us) if it is starting to condense, and forming denser regions?

EDIT: The actual paper says "photoevaporate"; Wikipedia explains: "Photoevaporation is the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. The term is typically used in an astrophysical context where ultraviolet radiation from hot stars acts on clouds of material such as molecular clouds, protoplanetary disks, or planetary atmospheres."