Answer: You may be thinking of barley straw which seems to cut down on algae blooms when packed into a bundle and floated in the pond. This should be available from local water garden suppliers or from catalogs such as Lilypons (they call it Ponstraw).
In a pond, there is usually a reason for ongoing algae problems apart from the temporary spring algae period that lasts a week or two. Algae can usually be kept to an acceptable minimum level by maintaining a good surface covering layer of foliage from waterlilies and other floating plants, avoiding overfeeding fish, and using ample oxygenating plants underwater to both oxygenate and use up excess nutrients in the pond water. Finally, make sure your pond is not overstocked with fish as their wastes (and excess feeding) can contribute to an algae problem as well.
Enjoy your pond!
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