Honestly I travel quite a bit, so I'm telling you Marrakech is totally safe to go to alone! That said, the usual stuff for any trip: don't flash your phone or camera around, stick to the tourist areas especially when it gets dark, and watch out for the "guides" who approach you in the medina because they're gonna try to squeeze you for a ton of cash! People are really hospitable once they like you, and October is an amazing time to go because it's not the hellish heat of summer. I personally really enjoy bird-watching in the surrounding areas, there are incredible species if you venture out to the rural zones a bit.