This is an allusion to Sponge Out Of Water but with My Little Pony characters. Grogar ghost was inspired by this.  
I am enjoying that headcanon where Twilight Sparkle sends the terrible trio to Luster Dawn and her friends to reform them just like how Celestia sent Discord to the mane six and also they will be fighting the real Grogar. I was in that opinion who does not care if the trio got reformed or not, but after the episode “Frenemies” showed me there is at least some hope for them and turning them into stone was too much. People say ‘but they refused again in that episode’. My counterargument is a bad person will not become suddenly good just because they did one good deed. Discord (before season nine) is an in-universe examlpe for that, he was in the heroes’ side but he was still a prick because he was still learning, after he betrayed them for Tirek he backstabbed him and understood truly what friendship is and onward he helped them in his own way. But then season nine decided to ignore all of that for some reason.
 
MY LITTLE PONY © HASBRO  
SPONGEBOB © STEPHEN HILLENBURG
 
Luster Dawn: Everything we know and love has been destroyed.  
Cozy Glow: Yeah, looks like they are gonna have to change the name of Ponyville to Dirtyville. Right, Luster?  
Luster Dawn: That is kinda gross, Cozy.  
Cozy Glow: Yeah. Yeah, too soon, huh?