It'll appear soon. Her calculations have already told her exactly when and where. Mercury hides.
The girl called Reika is attacked and becomes the youma Rikoukeidar. Jupiter and Sailor Moon fight her after some ridiculous speech about wanting new boyfriends. (If only. Ah, as expected, now Sailor Moon swears her eternal love for Tuxedo Kamen. Mercury shoots her an unseen angry look.) There's no need to interfere in the battle; let them fight for her instead.
Mercury waits.
Mars and Mercury - her alternate - come just in time. The newly-arrived Mercury's fog is thick. And there, there's Tuxedo Kamen fighting with Zoisite over the rainbow crystal that was hidden inside the reborn youma.
Mercury prepares.
The sandy-haired Zoisite tricks Tuxedo Kamen, but before he can grab the crystal, a jet of water knocks it away from him. The water turns to ice just as quickly, and the blue gem slides into the waiting hands of the Dark Mercury. She smirks as they turn shocked faces in her direction, but the fog is still thick and they both withdraw in the assumption that it had been Sailor Mercury.
Mercury wins.
Gathering rainbow crystals will be interesting for awhile. One or two from each world she targets, and the ginzuishou will never fall into anyone's hands, can never be formed at all, just as she wants it. It keeps that power from emerging to unleash its calamities, it strikes an indirect blow against the senshi, and it lets her lash out against the infinite endlessness of the nexus.
Mercury vanishes.
Back in her room of the castle, Mercury examines her prize. It will soon join several other blue crystals. There are more colors already, but she's avoided gathering the full range of the rainbow. Each crystal gleams with a luster deeper than normal gems, more vibrant and alive. They're strikingly beautiful, and she smiles, pleased.
She puts this newest one away, and Mercury sinks into the quiet brooding that so often follows the thrill of action. She sits cross-legged on her bed, eyes closed as though deep in meditation.
(ooc: 'Away' is still visible for anyone who may want to notice the crystal(s). Also, because Rico is epically awesome:
this.)