Bubble algae
Valonia ventricosa
Single-celled green algae that grows as shiny, taut spheres on rock — one cell per bubble, which is why it is unusual. It is slow and mostly cosmetic, but it shades and crowds coral it grows against, and it turns up faster in tanks with plenty of nutrient to spare.
Make sure that's what it is
Hard to mistake: glossy, water-balloon-like green spheres from a couple of millimetres to over a centimetre, firmly attached at a small base. Unlike a coral or a sponge it does not react to being touched.
What it looks like
No photos of Bubble algae yet. If you have it in your tank, a clear shot here is what the next person will compare theirs against — it's the most useful thing you can add to this page.
Getting rid of it
In the order we'd try them. Every method cites where it comes from, and where the reason a treatment works isn't actually established, it says so.
Emerald crabs — weaker than their reputation
Biological controlWhy it works
Seeded here as a qualified option rather than the standard recommendation, because the controlled study does not support the standard recommendation. Mithraculus sculptus does eat Valonia, and larger crabs eat significantly more than small ones. But offered a choice, the crabs took pellets over bubble algae 77% of the time and frozen mysids over it 69% of the time, and consumption dropped significantly when pellets were available. The authors' own conclusion is that the species "might not be such an efficient bio-controller of the pest V. ventricosa as previously thought", with the caveat that efficiency may improve if mysids rather than pellets are the tank's food.
What to do
If you use them, size up rather than down, and understand that a well-fed tank gives them no reason to eat algae. Expect suppression rather than eradication.