The algae on the rocks where the encrusting kind. Nothing can remove them (other than tweaking the water chemistry). If the Olive nerites wandered on a rock they'd scrape it clean as on the picture above. But it was a very random process - they didn't specifically head for the algae.
I don't have observations on how the Zebra neritinas do against algae. In any case the shell of the Zebra is much more attractive than the Olive nerite.