Has this background alien appeared anywhere else in Trek?

[UmberHulk]

It's from the episode In Purgatory's shadow

 

[Laura Cynthia Chambers]

Here you go: https://memory-alpha.fandom.com/wiki/Deep_Space_9_visitors#Alien_0209

 

[Finn]

That looks like Sito Jaxa.

 

[Lynx]

That looks like Sito Jaxa.

Well, Sito obviously survived (according to Memory Beta) so she might have visited Deep Space Nine on some occasion.

As for the aliens she is speaking too, it looks like the Creature From The Black Lagoon.