How should you address academic colleagues in the United States? Is the "Prof" title necessary?
In the US, you generally address colleagues of a similar status by their first names—so your fellow faculty members would definitely be addressed informally. This would of course extend to collaborators and acquaintances in other departments.

I think the only exception to this would be in the case of formal communications—an official memorandum or letter from a faculty member to someone in the university administration, for instance. But otherwise, in any oral or informal written correspondence, I'd stick to first names.

(In other countries, follow the appropriate local traditions, as they may be very different from US standards.)