What the achievement asks
Collect a thousand Prismatic Manapearls. Ten points, account-wide, both factions.
Manapearls are the Nazjatar currency. They come from world quests, from the zone's daily work, from treasures and from rares, and a thousand is a large number that arrives slowly.
There is nothing to optimise, only to not waste
This is a counter, and no activity in Nazjatar pays dramatically better than another. What actually decides how long it takes is whether you are in the zone doing its dailies at all.
The one thing worth knowing is that the count is what you have earned, not what you are holding. Manapearls are also the currency for benthic gear and for the cloaks, and spending them does not set you back.
That matters for Mrrl's Secret Stash, which needs you to buy Benthic Cloaks at five manapearls a go until you get the right one. Buy freely: every pearl still counts here.
It finishes behind everything else
A thousand manapearls is the slowest of the Nazjatar counters, and it is the one that finishes last for most people. That is fine, because nothing else in the zone waits on it.
Anyone working through Subaquatic Support, Nazjatarget Eliminated and Puzzle Performer is already doing the daily circuit that pays manapearls. Treat this one as the receipt for the others rather than a task of its own.