Deyika Nzeribe, 50, was a former chair of the party's Manchester branch and had stood at several local elections.
His campaign manager Astrid Johnson said party members were in "total shock" after losing a great friend and colleague.
The father-of-three and long-time green community activist, from Hulme, launched his campaign in October.

The plan also pledged to free up hospital beds by cutting the number of patients staying longer than they need.
And it set out commitments to recruit more GPs and nurses and regionalise some hospital services.
It follows Audit Scotland saying the NHS had met only one of its eight waiting time targets.
The public spending watchdog also said the health service needed to make unprecedented savings next year, and that NHS funding was not keeping pace with increasing demand on the service.
The government's Health and Social Services delivery plan said Scotland needed a "fundamental move" away from a "fix and treat" approach.
Instead, the focus will be on preventing people becoming ill in the first place, and then intervening as quickly as possible to prevent their conditions becoming more serious.
The key to this will be attempting to create a culture in which healthy behaviours are the norm, starting from the earliest years and persisting throughout people's lives.
The document also said mental health should be given equal importance to physical health, and stressed the need to address the underlying conditions that affect health.
And it said that, when people do need hospital care, the aim should be for day surgery to be the norm, with people who need to stay longer discharged as quickly as it is safe to do so.
The delivery plan said it aimed to reduce unscheduled bed-days in hospital care by up to 10% (about 400,000 bed-days) by 2018, through reducing delayed discharges, avoidable admissions and inappropriately long stays in hospitals.
Once they are discharged, patients should get "more appropriate care in a more homely setting", with good quality community care in place to ensure people only stay in hospitals for as long as they need specific treatment, the plan said.
And it also said that some acute local hospital services should be transferred to larger regional - or even national - centres of excellence, where patients can receive the best possible treatment.
The plans said: "Ultimately, individuals and where appropriate, their families - should be at the centre of decisions that affect them. They should be given more freedom, choice, dignity and control over their care".
Among the other commitments were:
Announcing the plan, Health Secretary Shona Robison said: "We want more services and more care delivered closer to home.
"And when someone does require specialist care in hospital we want it to be delivered in a centre of real expertise that is underpinned by our unswerving commitment to patient safety."
She added: "The plan I am setting out today puts actions and timescales to an already established direction of travel which we know has the broad support of healthcare professionals, charities and patient groups.
"It recognises that we must up the pace of change if we're to deliver modern, sustainable health services and that local health boards and integration partnerships have an important role to play in taking this forward over the next year and beyond."
The Green Party's candidate for May's mayoral elections for Greater Manchester has died of a heart attack.
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The Scottish government has published its blueprint for the future of the NHS, with a focus on treating people closer to home rather than in hospital.