The results after the first treatment was quite noticeable.
The issue could be coming from the live rock. The algae seems to be well rooted in your live rock which makes it even more tricky to eradicate. One way to find out if the issue is from the rocks or not is to put a new piece of live rock in the tank and see if you get the same algae growing at all.
Thanks! The tank is roughly 18 months old and I did not use live rock, just ceramic rock. The manufacturer (Aquaroche) guarantees that there are no phosphates in the ceramics so I can't see why could they be an issue...? Maybe having a too small cleanup crew allowed detritus to accumulate on the surface...?