If you can, remove the rock get it off and give it a freshwater dip if it's not too far cycled... (scrape, chip or scrub it off)...
If that's not doable, then use a syringe with kalkwasser & inject the little bugger til it goes away... it might be novel now, but when it spreads and you have a hundred that are killing things you'll wish you had done it now... 
__________________
135G Mixed Reef. Bullet 2, 25 gal refugium, 2 X250W MH + 4X 96W PC\'s, DIY Calcium Reactor, Coralife 1/6 HP Chiller, Phosban, Tunze, 2 closed loops & SQWD\'s, Seios, Coralife 4 stage RO/DI & a bunch of other expensive gadgets... I may never retire, but I'm gonnahavahelluvanaquarium!
|