Depends on the species of string algae, there are many. String algae that attaches to the surfaces of rocks, etc, has a hard time getting a hold on slick material like plastic and synthetics. Could be other reasons too.

The cloth could be treated with an anti algae product. Can you check the info that came with it or find it on the net?

Hi
I usually find that given enough time algae will grow on anything lol. I always laugh at the so called "algae
resistant adhesives". Maybe a month two tops lol Algae is sure not finicky, about all it needs is a steady source of moisture and even if that quits will go dormant and wait for good times. Report back in a month or two??? gary