Star dress is when Lucy gains the power of her Spirits and also their clothing style like a re quip. I initially thought that the first time she used it was in Leo's form after the TS but during tartaros battles when she summoned the spirit king, he granted her the ability to use Aquarius' magic in a similar way and her appearance changed as well so that was actually her first time of using Star Dress and she was a lot more powerful then.
What I'm interested in though, is why she can do it by herself now? It seemed like a special thing when the Spirit king bestowed the power on her. Also, could it be that she needed the spirit king to do it first before she could do it herself or was it just an ability she didn't know was possible or couldn't execute for some reason? This is a bit reminiscent of Dragon force which always needed external stimuli until GMG when we found Sting/Rogue could use it at will and Natsu later reproduced this against Mard Geer.
You can also discuss the Star Dress Potential here.
What I'm interested in though, is why she can do it by herself now? It seemed like a special thing when the Spirit king bestowed the power on her. Also, could it be that she needed the spirit king to do it first before she could do it herself or was it just an ability she didn't know was possible or couldn't execute for some reason? This is a bit reminiscent of Dragon force which always needed external stimuli until GMG when we found Sting/Rogue could use it at will and Natsu later reproduced this against Mard Geer.
You can also discuss the Star Dress Potential here.