Sure, this is one solution. But I suspect the 'culprits' use vegetarian because pescatarian isn't a well known term and invites more questions than it answers.

Vegetarian sums up a large chunk of their preferences, even if it isn't accurate.

If people just use the right word, and when someone asks "what does that mean?" they tell them, this exchange of 5 seconds is enough to bring awareness, if everyone who is a pescatarian took that short moment.

Right:

https://en.m.wikipedia.org/wiki/Vegetarianism