This is basically my experience with Malzahar over the years LoL. He can be a really strong champion and is overlooked always.

These masteries are basic and good, not much to tell.

Magic pen marks and ap quints are a must.

You saw well, scaling hp seals. They will be a life saver and a must as well.

Cooldown reduction Glyphs are a must for malza as well cause I found myself too many times running away for skills to get ready and die.

Always finish Malefic Visions first, it's your best skill. You'll score double kills with it in no time.

Call Of The Void is second best, damage and silence, still you have to be in sync with other champ to do it. Mastering timing it will save your lives too many times.

Nether Grasp is a good team fight skill. But, it will be your ally of killing and saving you from some ganks. All ways cast W and E before. Ignite can be cast while casting it with no problem.

Null Zone is good only for your ulti, it's will map the lane on team fight "you go there, you die"

The items are simple.

Start with boots or Doran's and hp potions

Boot upgrade is a good first choice. Chalice of Harmony is a fist item buy cause you need the mana gen and the the magic resist as well. Put your hat on next, then the rest.

Then upgrade your items and buy your last ones. Athene's stats are great for malza. Liandry's will make malefic a killing machine. Void stuff for any (futile) resistance. And your hourglass will seal your win, and your ap through the sky with some good armor. Don't forget to use the active skill well.

This is it. Have fun playing. Experiment with this and all your builds. Hope it will serve you well.