Sounds like the dreadful cyano. This is usually an indicator of rising nitrates or phosphates.
Do you use RODI for your water?
How often do you do water changes?
What kind of fish food are you using?
What kind of filtration setup do you have?
Staying on top of your water changes and using RODI water is the best way to prevent cyano, as well as using good food and not overfeeding.
Since you already have some, however, we'll have to narrow down any potential sources of excess nitrates or phosphates in your system. In the meantime, remove what you can by hand, and keep up with your water changes. There are a few things you can do before resorting to chemicals.
Also, post a pic of it to make sure it is what it sounds like.