L. pelta will eat many forms of algae, but seems to have a preference for the brown seaweeds.  The "tongue" or radula is rough like a file and  rasps algae from the rocks.

Reports on when L. pelta feeds are somewhat contradictory. Some researchers state that these limpets  feed only when submerged, while others claim they will also feed when damp or even completely dry! But then Martians studying our feeding behavior might come to contradictory conclusions.  

Side view of feeding limpet