Mongkok vs Mong Kok
One word or two? Well it doesn't really matter as neither is the original Chinese name and both are now widely accepted. As examples you will find that the MTR station in the district is the Mong Kok MTR station in two words, while the football stadium is Mongkok in one word.
Either way it doesn't match the current name. The word "Mong" in the original name may have meant a type of basket used for fishing, for in historical times it was a fishing village, or it may have referred to a type of fern that was similarly common while it was a countryside village.
However today the name in Cantonese has migrated to a similar word more correctly anglicized "wong" which refers to being busy and hectic but in a positive and pleasant way.
A few buildings in the area also use an older English name which is Argyle, as in the Argyle centre and indeed the original name of the MTR station.