It seems to be a diminutive of the word for 'moon', thus 'light' in Swahili but several languages are spoken in Kenya so it could be Bantu, etc. with another meaning as those languages aren't as well documented and researched. The only published research I found states that it means 'a bit of light' but didn't explain the linguistic or tribal root of the name, only it's occurance in Kenya.
There's also the name Mwali which occurs all over the African continent and could relate to a major god, companion of Luwe. Whether that has any bearing on Mwelu or not, I don't know.