Each time i did druid epic i saw other seahorse assist and if siren is passing by samething.Nothing new here.
Siren's Grotto is pretty buggy when it comes to this. The mobs get in a weird state where they blink in and out, usually shifting vertically, and sometimes in and out of the water. Sometimes you can target them, sometimes you cannot.
Yup SG mobs have always exceeded expectations and done the mysterious. Love when a mob is bugged and you stay in combat for hours on end.