Mistra Bay is a pebble beach located between Xemxija Bay and Selmun. It is more common among locals rather than tourists, to the point that many tourist maps do not even mention it. It’s not large and is usually overtaken by Maltese, so that you have to show up early if you really want to spend the day there. There are a few facilities in the area, such as a restaurant and a bistro just across the road from the bay. It might be good choice if you wish to experience a “Maltese day at the beach”, but my personal opinion is that there are far better choices in Malta. If you decide to visit it, it might be worth following the coastline to get to the next bay also; it could definitely provide you a good chance to take a cool picture of pristine crystal clear water (as seen in the map below).
Mistra Bay can be accessed via a detour off the main road leading from Xemxija to Mellieha. I’m not aware of public transportation taking you there.