Hey,
I'm Moroccan.. t he question of "brooks" forces me to answer...it's like dress what you want!! Marrakech is mostly known as a hot city!!
You can just go on..a mulim or not nobody will tell you a thing!!
You will be surprised when you're there!!
Take care and Welcom in Morocco!!
I'm not sure that "dress what you want" should be the answer.
British in particular have an unusual manner of dress when in hot countries (eg Spain) where some wear just shorts (male) or bikini (female) whilst out and about in town which looks improper and unsightly and should ideally be restricted to the beach.
The appropriate manner is to dress conservatively, no tight tops, no cleavage, no tight pants on females when outdoors. In hotels one never wears swimwear in the restaurant either at breakfast or dinner.