2 minutes is fine too since we've pre removed all that mass.
that growth is also plugging up the porosity of the rock system, the in/out and ability of that rock to express its detritus, so removing it and rinsing off the rocks externally is the right reboot here Im certain.
ill link below a sand rinse thread, take an hour to check it out since it shows other peoples work before you act. many of them had more corals to risk, this is a great time to practice full on control in your reef and it will never misbehave again.
even if your example rocks were covered in corals and that algae was in between, we'd still be doing the same stuff. this is the action to take if preventatives don't work, we wont accept an invaded tank if they don't work, we'll just have to cheat it into compliance then back to prevention if possible. its impossible to have an algae invaded reef with that approach, only the set-and-leave option makes invaded tanks.
when that set and leave option works, that's an ideal, its low work, and its how we all start in reefing (being uber careful)
but after losing a tank or three, or with the appro external lady pressures

we'll turn into gardeners/internal locus of controllers mighty fast!
the sand rinse thread reference:
https://www.reef2reef.com/threads/t...ead-aka-one-against-many.230281/#post-2681445
per that thread, your exact breakdown and reset up would be:
catch fish and hold in buckets sep from rocks, not with rocks
hold your clean up crew and a few corals if applicable in there, but not rocks, we don't know how much waste is in them yet so they and the sand stay isolated from the sensitives.
step one is all sensitive animals in a bucket away from the tank.
with that done, all that's in the tank is bacteria (w be preserved) and rocks and sand and water and algae, we're free to work.
start by lifting out rocks and scrape off algae that algae, brush it off roughly, whichever. be rinsing rocks only in saltwater, to wash away that mass as you scrub scrub
when each rock is cleaned, mist them with peroxide, around any targets, and let sit a couple mins. rinse off last time w saltwater, they're now detritus and growth free rocks ready for install
whats left so far now is only muddy water and sand in the tank. either replace your entire sandbed with new, if its cheap and you want to, or just rinse your current sandbed any way you want to rinse it until its clear, and this w take a while. you will see in the reference thread I rinse my living, old sandbed with tap water for about 30 mins straight till it runs clean
*we pre rinse any live sand, before use, as the point of all those pages* when doing a bed replacement. Your sandbed is so new it doesn't have to be replaced if you don't want to, just blast it into compliance. This is the work price of having a zone in a reef that we can't access daily for cleaning (though many nowadays do pre stirring of the bed routinely)
then I saltwater rinse, then put the sand back. I didn't put back tapwater sand into my tank
use distilled water, or seawater if you want, but those are finite and tap isn't. tap rinsers can be thorough, saltwater rinsers always run out of mix water before its cloudless/murphys law
the sand retains its bacteria, so did the rocks, because these steps are not completely antibacterial.
reinstall everything with totally new water, re acclimate animals, the reef is reset. we'll put pics in our threads if you do this work.
anticipating things that can go wrong from prior posts:
-incomplete rinse. detritus is the source of your cycle risk, so don't leave some in the bed. be thorough, even replacing the entire bed is advisable if you want, cloudy sand has nothing we need to keep or transfer over. make a cloudless sandbed before you put things back on it.
-don't rinse rocks in anything but saltwater, they're our filtration source we're protecting. the light peroxide misting for 2 mins wont hurt em
-storing pre cleaned rocks with fish. one poster lost fish as the rock was emitting ammonia and had pent up detritus in it. holding separate stops that.
pretty much those two were the only bad outcomes ive seen. hope that helps. I know its a lot of type but it took years to condense all those moves into actions that do not kill tanks.