Bryopsis. The bit on the crab shell is a dead giveaway. I tried hydrogen peroxide but didn't care for the results at all. I ended up using Reef Flux capsules and it worked really, really well. Surprisingly well. The last time I dealt with bryopsis was 2008 and it was a nightmare inducing magnesium based treatment. You can try the hydrogen peroxide but the whole tank Reef Flux treatment was cheap, simple, safe and took all of 15 minutes to order, read and treat the tank.
IF I were to treat again, I would go to
@the fish store and buy the liquid Flux Rx drops by Blue Life that they had in stock a few weeks ago. I know, I know, I just got done saying how simple and easy the Reef Flux caps were but the powder was a bit annoying to mix up and dose so next time I'm going to try the liquid product.
In the past bryopsis was the most-feared algae you could get but now it's super easy to deal with. I'm sure in 2028 there will be some fluconazole resistant strains but as things are now, bryposis is a moderate threat that's simple enough to deal with.