Early detection of invasive species is critical to their successful management because control methods can be enacted before the invaders become established. But what happens when an invasive species escapes early discovery because it closely resembles a native species? Marine algae in the genus Caulerpa are prolific invaders of tropical, subtropical, and temperate coasts, where they substantially disrupt ecosystem functioning. Caulerpa species including Caulerpa cylindracea are easily discernable by their characteristic rhizoids, stolon, and fronds (Figure 1a). However, a previously undescribed form of C cylindracea (Figure 1b) displays features that are atypical of the species: namely, an unbranched habit and the presence of long, vertical filaments, 10 times as thin as those typically seen in this taxon. These filaments are nearly impossible to identify visually in the field and, when detected, can be easily misidentified as native species in genera such as Derbesia or Chlorodesmis. Genetic analyses confirm that these filaments are C cylindracea, which is likely the most rapidly proliferating invasive macroalga in the Mediterranean Sea. What promotes such morphological divergence within a species? Preliminary lab-based experiments suggest that it may be a response to stressful conditions. It is unclear if other invasive Caulerpa species exhibit the same degree of morphological flexibility and whether individuals displaying the uncharacteristic filamentous morphology could revert to the regular branching morphology; although both morphological forms have been simultaneously observed at the same location in the field, the reversion process has yet to be observed under controlled laboratory settings. What is clear, however, is that this novel form can disperse widely, as its filaments break easily and can float over long distances to colonize new areas. Managing invasive flora is already challenging, and is only further complicated when they may hide in plain sight among native flora.