'must be off' is a correct and usable phrase in written English.
It is often used to express that someone needs to leave. For example: "I must be off - I have an early morning meeting tomorrow."
Outdoor lighting must be off at sundown.
Sadly, Gilbert & George must be off: artistic duty calls.
He adds that we can chat until two o'clock, when he must be off to Burbank.
The children will come to the station with me; and now I must be off".
Crittendon acknowledged that his timetable must be off, but could provide no evidence to support his memory of the vivid exchange.
Mine's all right, but I don't like the look of yours.' (A sniffy pause) 'Well, I must be off.' 'Yes, you're not looking so good.' 'Cheerio.
Any access to the BBC transmissions must be off-air.
"It must be off-putting to email your boss and ask for the day off.
Three constructs caused lethality to both sexes within D. melanogaster (Table 1), which must be off-target effects as msl-3 is only required in males.
The power must still be off when you do this to prevent injury and shocks.
All ornamental fountains must be shut off.
