Linda Key, 53, of Rochester Hills, and her husband, Glenn, 52, said the blob, which is about the size of a basketball and weighs nearly 2 pounds, was first spotted Sunday among the rocks lining their flower bed, the Detroit Free Press reported Thursday.
"We didn't know if it was going to start walking away or what. It was so weird looking," Glenn Key said.
The couple's son, Wyatt, 13, likened the object to something out of a sci-fi movie.
"The whole experience has been very weird ... looking at it, thinking it's an alien egg," he said.
However, mushroom experts identified the object as a fungus commonly called a puffball.
"I've seen those at least twice that size," Michigan Mushroom Hunters President Phil Tedeschi said. "They'll get up to like, maybe almost 3 feet tall."