If it's a soft coral it's likely just shedding, and if so that's how leathers/softies grow, if it's on a lps/sps it could be a vermited snail web or a leather shed, could also be mucus from irritation or feeding, unlikely but could be a bacterial infection, if it's a shed or vermited web and it's an LPS or SPS then I would blow it off with a pipette, if it dosent go away in a day or two then write an update