Ward told Greenpeace online he had taken "great pleasure" in removing its work, adding: "Thought Greenpeace was all about leave no trace and you do this to the mountain side."
Bangor Conwy Môn Senedd member, Janet Finch-Saunders, accused the group of "environmental vandalism", saying burnt hillsides should not be used "as a canvas for political protest".
Greenpeace UK's co-executive director, Areeba Hamid, said: "Our artwork was not intended to disturb [people's] grief but to draw attention to the escalating climate crisis that's putting these beautiful places and communities at risk.
"We took great care to minimise our impact on the land. The artwork was temporary by design: created with a water-based paint often used on sports turf and safe for grass, plants and soil.
"It was applied to an area where the fire had already burned away all living vegetation, leaving just rocks and ash."
The threat, Hamid said, had not been "a dab of water-based paint" but "extreme weather driven by fossil fuels".
She added: "We wanted to challenge the system that caused this damage and send a vital message to the prime minister: the science shows that more oil and gas drilling means more communities left to cope with the devastation brought by heat, droughts and wildfires."
