[Sk8r]

Ah, where was I? About the 3rd month, give or take a month: bubble algae. Why am I not surprised to find it is a eukaryote?

eukaryote? you mean---that strange reference to a passing remark in bioscience? That blue-green stuff that re-oxygenated our planet after something offed the giant reptiles in the Permian?

Ohhhh, yeesssss. Eukaryotes. ewe--carry-oats. Primaeval life. Well, hey, we ARE building a small ocean here, are we not?
Meet the eukaryotes: bubble 'algae', cyanobacteria (not an algae), dinoflagellates (not quite an algae---or an animal) and diatoms (not an algae either).
Characteristics: really ancient. Before the dinosaurs. Cyano gave rise to the chloroplasts in every living species of green plant. Dinoflagellates gave rise to the zooxanthellae in your corals, the little food-producing units, the brown slime that insulted corals give off when sunburned, etc.
Further Characteristics: HARD to get rid of. They do not respond well to things that do not kill your tank...they don't respond to refugia, OR to phosphate reducers, because, well, take cyano for an example. It thrives on light, carbon dioxide, and nitrogen (which makes up 90 percent of our atmosphere). See why they're a pain? Diatoms thrive on silicate. (a ro/di filter DOES remove silicate from your water). Cyano can be done in by turning the lights out for 3 days once a month (4th day actinic, to avoid sunburning your corals). It takes time. Valonia---when you find out, you tell me. Clearly it's thriving on something. And dinoflagellates are probably the hardest to get rid of.

They're not algae. They're not animals (bacteria) either. Dinos swim. Cyano crawls. Is that creepy enough for you? This stuff is Audrey, from Little Shop of Horrors.

It'll show up as green pearls on your rocks (valonia); or as red/brown (depending on your light color) stain on your rocks/sand (cyano OR diatoms, which are a bit browner); or as a more basic brown (dinoflagellates). None are a happy discovery.
They are all a pita to get rid of.

Cerith snails will eat diatoms. A diatom filter can get diatoms, if they are stirred up. But then a diatom filter can get most everything Else in your water column, like your copepods, which is not desirable to do too often. And a ro/di filter can tame that plague.
Emerald mithrax crabs are alleged to eat Valonia.
And cyanobacteria will respond to the lights-out treatment.
Dinos---probably a diatom filter would help.

What NOT to do: high risk factor, and don't tell me you survived it: so did I, but PLEASE don't do this in your first few months of tank life:---Red Slime Removers come in several brands, but they are basically antibiotics. They kill bacteria and bacteria-like things like cyano. They kill it---and part of your sandbed and rock bacteria. And send all that biomass to the skimmer at once. if you have a really good skimmer, and follow instructions, your tank may survive.. If you're not lucky, your tank will suffer a nitrate/ammonia meltdown and everything in it will die. This is called a tank crash. This is why I say don't use this treatment when your sandbed is still new. It's far too rough. SOme people are lucky/have good skimmers. But don't bet your tank on it.

The eukaryotes show up in about your third month. And they WILL show up periodically whenever your tank feels like it. Keep your nutrients in line, do your water changes, and when it does happen, don't panic. Your stuff is still white sand and living rock underneath that disgusting red sheet: it will disappear---eventually.

The only treatment I recommend for these things for newbies is cerith snails, emerald crabs (they're cute, anyway) and lights-out. Eventually if you have a killer skimmer and you feel uncommonly lucky, and are sure how to follow those instructions meticulously, you can try the chemical treatments, but I haven't. Once was scary enough, and it cost me 300.00 in store-bought pods for my mandarin---they all died out, and I had a fish to keep alive.