It's rather difficult to tell what a chiton or limpet is doing by sitting
there watching it.  They do feed on algae (microscopic forms which grow on
rock surfaces), but they are very slow-moving animals, and most of them are
nocturnal feeders.  The few times I have observed living molluscs at night
with a flashlight or lantern, the light didn't seem to disturb them, or
deter them from what they were doing.  The best way to observe chitons or
limpets feeding is to place them in an aquarium.  They will adhere to the
glass, and feed on algae that grows there, and the feeding action can then
easily be observed or even photographed through the glass.  More active
molluscs, like top shells and periwinkles, can be observed feeding in
nature.  Not only are these types more active, but their heads are visible
(unlike chitons and limpets) - a definite advantage in observing feeding
behavior.  They frequent rocks and seaweeds, but do not generally feed
directly on the large seaweeds on which they are found.  Rather, they feed
on diatoms and other microscopic algae which coat the larger seaweeds and
rocks.
Paul M.