beginning of GHA or green cyano...see if it comes off easy with a turkey baster or powerhead. Any algae that can grow on or in between your corals will hurt them. Check your lighting, nutrient levels. Do you use any P04 control?
What are your water parameters? List all that you have tested but important ones related to algae are phosphate and nitrates.
What type of filtration systems are you running on your tank?
Do you have a TDS meter for your RODI filter ? If so what is your TDS reading on the water being produced by the RODI filter?
How old is your tank?
What type of lighting do you have?
How many hours a day are your lights running?
The best way to resolve algae issues is to eliminate the source of the problem. Corals can be impacted by algae but if water parameters are out of acceptable levels it can also impact corals.