This "monster" looks much larger than the "length of the palm". However, it is necessary to recognize that this is an unusual representative of the animal world, which in some people can cause a feeling of disgust. His image would be perfect not only for alien invasion, but also for horror films :) I do not think that it represents a real threat to health ... probably :)
I remembered a story that happened in my country in 2010. In one city people began to dissolve rumors - that in the river wound up a monster (40 centimeters in length). They claimed that this "monster" attacked people and left serious wounds on the body. Here he is...

As a result, it turned out that this "monster" is a shin, an absolutely harmless crustacean being. He does not eat people and eats seaweed :) Here is an example of how people can in their imagination turn an inoffensive crustacean into a monster :)