Brown Slime - Possible Blue-Green "Algae"

[Delphinus]

Well, I'd say you've nailed the diagnosis, sounds like a classic case of cyanobacteria. Despite the name it can be just about any colour as well as blue/green (most often red, thus it's other nickname "red slime").

There's a couple of things to consider here. Basically, it will take off and do really well in a tank if there are certain conditions met. Most often, a buildup of excessive nutrients (nitrates and phosphates), also low O2 saturation, low flow, low or no skimming, old lamps or lamps with poor spectral output.

Upgrading your skimmer will help, no question. Also will increasing flow, and trying to manage your feedings to prevent an excess of uneaten food contributing to the buildup of nitrates.

You can also add a refugium where you can grow macroalgaes such as caulerpa, these will compete with the nuisance algaes in the main tank for the excess nutrients. It might not solve the problem outright but it can certainly help you out in dealing with it.