That looks like the start of Bryopsis algae. I've used Flux RX that has Fluconazole in it that kills the Bryopsis. Also, you can physically remove it, just make sure you don't have a strand get loose and plant somewhere else in your tank.
I'd watch it over the next week to see how it grows. If it looks like Bryopsis then it is. I haven't found a critter for the cleanup crew that really goes after this stuff. A