Though not actively aggressive, Krocusses will do damage upon contact. Neither beam weapons, Missiles, nor Morph Ball Bombs will do damage unless aimed at the lower part of the Krocuss’ back when its gills are open. However, both the Boost Ball and Power Bombs will do damage regardless of where they are placed and whether the Krocuss’ gills are exposed. The latter weapon will kill it instantaneously.
A single shot of the Light Beam on its exposed gills is also sufficient to kill the Krocuss, as it will cause the creature to burn until its health is depleted. Super missiles will also kill them with a single hit if aimed correctly.