Probably best not dosing anything yet then, see how you go with manual removal, and a good cuc, it will take time though.
Really important to remove as much as possible as any that is left in the tank and dies will just release all the nutrients back into the water.
Use a small sand siphon, in the same hand pull the algae off and straight into the siphon. If possible better to remove the rock and scrub it off, your corals should be ok for a good few minutes out of water
Remember that your tests will be incorrect whilst the algae is there as it will be using the PO4 and the nitrates.
As said you really do need to work out how it got out of control to stop it happening again.
Hard to give exact advice as no picture or details on the tank for more individual advice.