Mad Hatter is a corrupted, zombified alien. His body is entirely covered in hardened mercury, and his blood is also mercury. He has a thermometer-like container on his back that allows him to shoot mercury. He wears black gloves that go up to his elbows, black boots, and brown pants. He has two strands of hair, presumably also made of mercury. The Simplicitrix resides on his chest.
- Mad Hatter has the ability to control mercury, a highly deadly material. He is able to manipulate this in many ways, such as through direct contact (as seen when he grabbed ElevenEleven's wrist) or by shooting it (as seen when he shot it at Techadon 59260 but also accidentally hit Jessica's leg, showing that Mad Hatter's mercury is able to go through her suit, which was supposed to be completely impenetrable when worn in full.
- Mad Hatter's mercury is strong enough to cause instantaneous effects.
- Mad Hatter is immune to mercury.
- When Alan transforms into Mad Hatter, his blood is turned into mercury. This means Alan has mercury poisoning when he transforms into a different alien. Alan was seen coughing up blood after transforming back into human, and also had his skin cracking when his blood became mercury.
- If Mad Hatter isn't careful, he can accidentally hit his friends with mercury, as seen when he accidentally hit Jessica's leg with mercury when first using the alien.
- Ouroborosaurus species, Mortuus Est Piscinae, are immune to mercury.
- Due to having fought Techadon 59260, he can no longer be used against him.
- Mad Hatter accidentally debuted in The Annihilators Part 2 to fight ElevenEleven, but accidentally gave Jessica mercury poisoning in the process. The selected alien was NRG.
- The Annihilators Part 2 (debut; accidental, selected alien was NRG)
- Mad Hatter's name comes from mad hatter disease.
- Mad Hatter's species name is a play on the word "mercury," and the YouTube handle of Mark Fischbach, "Markiplier."
- Mad Hatter's species name is a play on the Latin words "morbus," meaning disease, and "mercurius," meaning mercury.
- Mad Hatter's art was made by SsS.
- Mad Hatter was the featured alien for January, 2018.