Prismatic Manapearls are a new currency added in patch 8.2 of World of Warcraft, and this currency is tied to activities in Nazjatar. This currency is used to buy the new Benthic ilvl 385 gear from the faction hub (Finder Palta for Hide, and Artisan Itanu for Alliance). This gear is meant as a method to catch up with the players who already have this ilvl on their gear, but a great thing about them is that they have cool secondary effects that is directly tied to the Nazjatar zone.
The base Benthic item cost 5x Prismatic Manapearls, but can be upgraded to go as high as 430 if you got a lot of Prismatic Manapearls to spend.
There are also two different mounts that cost 250x Prismatic Manapearls each (Anokan Waveray for the Alliance, and Unshackled Waveray for the Horde), a pet for 40, and several difference Essences for salt at 5 to 75 Prismatic Manapearls each.
So, now the question is simply, how do you get these Prismatic Manapearls? The answer is pretty much any activity in Nazjatar. This means doing daily quests, world quests, finding hidden treasure chests, killing rares for the first time each day, completing rare encounters and so forth. You will be able to get a certain amount of them every day, so if you want a full set of 430 items, you will have to do a lot of Nazjatar content for a good while.
Most daily quests and world quests can award a single Prismatic Manapearl, while the rare mobs have a chance to drop one in the first kill per day.