It's the start of some kind of macroalgae, too early to identify the species. if you have tangs, they will eat it (free food!). Macroalgae is good for the tank, however it can get out of hand if you don't keep it pruned back (called harvesting) and as you prune it back you remove nasty stuff from the water, since macros use phosphates and nitrates to grow. Most people with coral don't want it in the display because it will cover corals. For fish only systems, it adds a little color and helps keep parameters pristine.