This small building block is a slope corner with its cusp (pointed end) pointing and fitting to one corner of an adjacent cubic block. It is not pyramid-shaped.
The grey slope corner looks like being made from plastic or metal and feature small dark indentions at their "base".
Galactic Hull Slope Outer Corners were implemented with update R46 (Galactic) in September 2017.
How to obtain Galactic Hull Slope Corners
No crafting recipes have to be unlocked for this.
How to craft Galactic Hull Slope Corners
Galactic Hull blocks are part of the Galactic Super Bundle that was implemented with update R46 in September 2017, and can only be crafted (default crafting key is Q) in the Crafting Menu after buying this Super Bundle in the Store for Coins.
Cutting the blocks and slopes in a Processor does not require any crafting recipes.
Buying the Galactic Super Bundle in the Store for Coins is the only way to permanently add the (already unlocked) crafting recipe for Galactic Hulls to the Crafting Menu, where the recipe will keep being available even on game worlds where the world option "world bound recipes" is enabled.
How to use Galactic Hull Slope Corners
These slope corners can be placed for building and decoration purposes. They fit well together with Galactic Hull Slopes and can be used as a completion at their sides, like for roof corners.
Galactic Hull Slope Corner blocks can be rotated by pressing and holding R (as the default key) and moving the mouse while holding the left mouse button. The chosen rotation angle can be "locked" too by simply typing "r" while looking at a rotated block or item, so that all items of the same stack will be placed facing the same direction.
Like all (even non-cubic shaped) building blocks they can also be displayed on Placemats, Stone Wall Shelves, Galactic Grav-Chambers, Hidden Temple Altars and similar display "cases". The slope corners will be automatically reduced in size by these decoration objects.
Galactic Hull Slope Corners aren't used as ingredients in any crafting recipes and cannot be processed any further in Crafting Stations. They are non-flammable and like all other crafted objects, these building blocks are also non-corruptable.