Does it look like snot, and is it growing from the bottom up? If so it sounds like dinoflagellates. This can be caused by excessive nutrient build ups. There are several things you can do if this is the case.
Big frequent water changes, cutting back on feedings, lessening the photo period and siphoning it out can help and maybe get rid of it.
I have heard you can essentially turn the dino into a nutrient export too. This is done by MAJOR water changes every couple days, adding some macro to compete for nutrients, and increasing the photo period to 18-20 hours. This will cause the dino to grow, (obviously) and use up more nutrients. Then as it gows siphon it out and hand grab it from the bottom. As the nutrients are reduced the marco will eventually out compete the dino you are constantly removing. You also are removing excessive nutrients as you remove the dino. I have never tried this, but if I get an out break of dino I will, it makes more sense to me to use the dino as a nutrient export itself.
Finally, try a search for dinoflagellates. A lot of reefers have battled this. There are more methods out there, it's just a matter of finding one you like or want to try. Good luck.
