It produce visual and auditory hallucinations similar to the effects of LSD and DMT.
Tripping on the salema porgy was actually a popular past time during the Roman Empire, which would explain how they come up with monsters that looked like this.
The reason salema porgy can cause hallucinate might be because of their diet, which consists of a special algae Caulerpa prolifera and single cell plankton, both of which are carriers of DMT.
A 40 year old and a 90 year old consumed the same Salema porgy. Both ended up in the hospital with digestive problems and horrible hallucinations that lasted over the period of 36 hours.