Hello! You've got yourself an emerging algae outbreak! Looks like green hair algae covered with cyano to me, but I could be wrong. "How do I get rid of algae" is the million dollar question in reefing. BRStv has some great videos on it, but basically you need to improve your nutrient export (keep nitrate and phosphate low to starve the algae) and manually remove as much as possible. An appropriately sized protein skimmer and refugium will help a lot with nutrient export. There are tons of products to address algae issues and everyone has had success or lack there of with each product. If the slime becomes dominant chemiclean is a good way to go. Vibrant can also be really useful.
If you post some more details about your system you can get some more specific recommendations. Cheers!