You would change the subject using the STROVE method:
Subject: I (Ni), You (U), He/She (A), We (Tu), You plural (M), They (Wa)
Then fill in the rest of the sentence: Tense: Present Perfect (Me)- ('Have/Did') Relative: Not used Object: Not used Verb: (Ku)lala- ('Sleep') Ending: Je- ('How?')
So how did he/she sleep would be: amelalaje
My question would be whether one could answer "Not terribly well, because there was a fly in my bedroom and it kept buzzing in my ear all night." If that's a possibility, then it is a real question. If anything other than "Peacefully" would be odd, then I would simply translate it "Good morning, Juma." I find that some people translate European languages according to meaning, but African and Asian ones quite literally, which makes them seem more exotic, or even primitive, than they are. It's the same way people used to translate Latin and Greek into archaic English, just to make them sound old in some way.
Well, going off of the fact that (according to google) Umelalaje is often used alongside Habari za asubuhi ("Good morning"), rather than strictly instead of it, I'd say it really is a question. I'm no more familiar with Swahili than you are and can't say if it's frequently answered with something other than "peacefully," but I'd imagine so.
Firstly, I am from the United States where English is the native language for most of us and we use this phrase often. "How did you sleep?" is not only acceptable but polite and common to hear, especially in hotels or when staying at a relative's home.
Secondly, we're learning KiSwahili here... not English, so, you shouldn't focus that much on the English equivalents as long as you are getting the general understanding of the KiSwahili phrases.