rescuing a tank. please help!

[asylumdown]

It depends on what kind of algae it is, and what sort of conditions are in your tank. Algae won't grow without available nutrients, but it's really hard to tell what your available nutrients are some times. Does other kinds of algae grow in your tank? As a general rule if other species of algae are actively growing (even if your phos and nitrate tests show low or undetectable levels), then there's enough available for all algae to grow. If your rocks stay generally bare even without snails/herbivores, then you likely will have a hard time getting any kind of algae to grow.

However, regardless of your nutrient dynamic, you can't have a problem algae unless you introduce a problem algae. Just like you can't have a problem with aiptasia unless you introduce aiptasia, and you can't have a problem with majanos unless you introduce majanos. If the species that are on that rock are alien to your established tank, who knows what they'll do. And if your refugium's water is connected to your main system's water, they certainly won't be polite and stay put. Most aquatic algae releases gametes and spores directly in to the water column.

Obviously it's up to you, but what benefit do you hope to get from that rock that you couldn't get with a little patience without all the potential problems?