Slimecraw is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Slimecraw (unique) style.
The 1-piece set bonus adds 833 Weapon Critical and 833 Spell Critical.
2-piece set bonus: Gain Minor Berserk at all times, increasing your damage done by 8%.
The Shoulder can be acquired by using an Undaunted key on Maj al-Ragath's chest. The Shoulder type, weight and trait are RNG based and can be from any dungeon this NPC gives Pledges for.
The Helm can be acquired from Wayrest Sewers I which is part of the base game and located in Stormhaven. The Helm is guaranteed to drop from the final boss in veteran mode only. The weight and trait are RNG based.
You can mix-and-match Monster Set shoulders and helms from different sets and add two 1-piece bonuses to your build, or wear a complete set to take advantage of the full 2-piece set bonus.