Two points.

1- to answer the specific question, the product you're asking about is often called "carbon assembly paste" and is OK to use on an aluminum post. There are two kinds, on uses grit one uses rubbery particles and doesn't scratch. You can also use lapping compound which you can beg from your local auto mechanic.

2- MORE IMPORTANT

Greased aluminum posts shouldn't slip. The fact yours does implies something else is going on. You can diagnose the specific issue with a quick eyeball check of two possibilities.

1- the cut slot in the frame tube is typically 1/8" or 3mm or so. If the post fits correctly the top shouldn't pinch closed much and the gap should be only a bit smaller than at the bottom. If the gap it the top is close to 1/16" when closed, odds are the post is undersized.

2- clamp bands often fail, not always by cracking. Look at the back of the ears and see if they touch (or even come close) you won't be able to clamp the post any tighter. If so, the only solution is a new clamp. Look for one where the ears transition to the band without an angle or notch (like this -- not this).

Lastly, if the post and band check out, you might solve the problem without buying anything. Remove the post and wipe it dry. Then wipe out all the grease to a depth of one inch or so in the frame. All the clamping happens in this top inch, so if you have it dry there you can still have grease below. Just be sure that as you insert the post you don't push it in too far and pull it back up, since that will smear the grease where you just wiped it out.