Informally, it's known as the Nexogon.
Bricklink calls it "Plate, Modified 6 x 6 Hexagonal with Pin Hole". The part number is 27255. It's original use was as a shield in the Nexo Knights line in 2017, but it has appeared in a few other sets since then.
If you're interested in ways to use this part, New Elementary did a whole series of posts from different ...
What you are seeing is actually the result of poor lighting, somewhat worn bricks, probably camera app noise reduction, and perhaps image compression, although if you look really closely you can kinda see it's a little darker where the lettering is.
As the creator, I promise that every piece in the hat is 100% genuine LEGO pieces (no knockoff brands). ...
As pointed out by others, the first three minifigures are genuine LEGO (with the second having perhaps a few custom accessories). The fourth is not a LEGO minifig. It may include some genuine LEGO elements (legs, hands, etc), but the militaristic accessories are not LEGO elements. This appears to be a custom minifig sold at one time by Modern Brick Warfare.
It looks like a possible match for these doll accessories.
"This listing is for a Barbie coffee shop or bakery accessories. Included are hot chocolate mugs, a coffee to go mug, croissants, muffins, donuts, a to go box, and a bakery display plate."
I see that this popped up on HNQ, so I wanted to expand on shoover's excellent answer with a build showing one way that this could be put together for folks that may not be as familiar with LEGO elements as the Bricks community.
As was already pointed out, the key element here is the 2x2 turntable base without a turntable attached:
Here's how you might ...
These came from a number of sets, including a handy "Decorated Elements" set that covered most of the printed parts from the mid-90s space theme.
However, Recon Robot appears to be the only one that contains two of the 2x2 tiles and only one of the 2x1 tiles.
Instructions can be found in a number of places, including on Let's Build It Again.
It looks like part of the old 9-Volt motor that was made from 1990 and 2002.
The full thing's part number is listed as 2838 which I assume is the mold ID stamped on dark grey base, while 2837 is the mold ID of the top part.
I don't think it's supposed to come apart, it looks like there are 4 clips on each sides which have been broken off of your piece.
You might be thinking they have the normal holed stud on top, which would indeed be a bit smaller. But instead what is done here is that they put the parts upside down, I think. So it's the bottom opening of the standard round 1x1 brick that you see on the top there.
They likely achieved that by using a 2x2 round tile with a stud hole as base, rather than ...
Looking at the pictures, I think Syberion has correctly identified the part.
Just want to cover the moment where you mention part modification may be required. I don't think there is a need for modifications, it should work with existing parts like this:
Additional parts are:
Bar 1L with Clip Mechanical Claw
Plate, Round 1 x 1 with Open Stud
Lever Small ...
These are Panel 4 x 4 x 6 Corner Convex Ribbed
They were mainly use as decorative pieces on corners in classic space sets.
In light grey they came only in one Set: Solar Power Transporter
(photo from Bricklink)
It looks like a "short shot" of Minifigure, Utensil Bottle, Erlenmeyer Flask with Trans-Purple Fluid Pattern (pictured below) which is included in the set. I would even say it lacks the Trans-clear "coating" around Trans-Purple piece of the combined element.
Short shot - is a term used in LEGO community to describe an uncommon occurrence ...
It's 99563 LegoMinifig, Utensil Gold Ingot (Bar) in Light Bluish Grey.
Originally used for Gold pieces, nowadays regularly found as bricks and other wall decorations, for example on the ground floor of 10251 Brick Bank.
In a classic rubber duck moment, the solution has dawned on me minutes after asking the question, while browsing the list of sets owned by me to find the one that included this piece...
This is ASSEMBLY ELEMENT Ø3.2 etc. or with Bricklink terminology: Plant, Tree Palm Top:
The complete lack of markings of any kind is still puzzling, but it is reassuring ...