A final charge alleging she made a misleading statement to officials was not accepted by Judge Jane Farish.
Fletcher said marimo - Japanese for "ball seaweed" - is small, green with a soft velvety appearance that is widely distributed in the Northern hemisphere, but there is no natural distribution in the Southern hemisphere.
Moss balls were declared to be a new organism in 2006 and prohibited under the Biosecurity Act.
During an interview with MPI investigator Richard Notley, Miranda admitted buying moss balls on AliExpress.
But she denied knowing it was illegal, and if she had known, she would never have done it.
There was no way she would harm New Zealand's waterways, she said.
However, she accepted that she "unfortunately didn't make any checks" before bringing them into the country.
Marimo moss balls have the potential to ruin New Zealand lakes and rivers, Judge Jane Farish said.
Only a very small particle is required, given that the algae can multiple very quickly in the right conditions, the judge noted.
Judge Farish said if Miranda had taken proper steps to check the status of marimo moss balls, then she may have had a defence to the charges.
Miranda will be sentenced on June 20.
MPI's Jonathan Cowie told the court that three moss balls were still unaccounted for. Miranda's lawyer Nicola Hansen said there was just one that hasn't been recovered.
Addressing the media, he asked anyone who has purchased moss balls to bring them to MPI and to not dispose of them down waterways.
Even a small filament, Cowie said, can lead to a moss ball growing and creating a "didymo situation".