Many dev give what looks like paint but its some sort of white powder (chalky cheapo coating cantonese say fui sui) or never put proper sealer and paint on coz it "looks white can liao". Enough to tahan until DLP warranty period can liao cos its done deal and "not my house syndrome" of the bangla painting it.
Suspect that crappy "white powder" has loosened and took the grey paint off.
This crap should be sanded or brushed out until its no longer chalky (ie basically should remove until bare cement plaster).
Then put on a suitable (in this case something that says OUTDOOR sealer) like the nippon 5400 (or 5100), don't remember which is which but u can check.
After the OUTDOOR sealer is on, put on some OUTDOOR paint like weatherbond or weathershield of your choice. Interior paint don't resist mould that well.

If not, it could be the aftermath of rainwater dripping on it (acid rain, mineral deposits.. think of it as flatter mini stalagmite), if brush off with stiff metal wire or stiff plastic brush it should reveal back the grey paint.

If you guys have a jet spray i would recommend spraying the wall to clear of the old paint as much as you can until you see the bare plaster wall. This is only done on the outside, where rain and shine will 100% hit and exposed. Nippon or any other brand doesnt matter, as long as you buy Weathershield. I usually use Dulux/ICI paints. So far good.

Need a higher bar maybe, im using 160 bar heavy duty jet spray. Because without that it sure wont come off and the job may look nice after finish, but it will wear out faster than advertised. Also use the same weathershield sealer and weathershield paint brand. I used to use Nippon sealer and ICI paint. hahaa. Not that it doesnt work, but better la, and buying all the same brands is easier also. I usually stick with Dulux/ICI.

Step 1: Jet spray and patch holes after. Make sure holes are nicely patched and no unevenness.
Step 2: Apply 1 coat of weathershield sealer, dont mix with water as stated on the paint pail, unless you have left it in the sun and it feels hard to paint then add just a little bit of water. Like less than 500ml.
Step 3: Apply 1 coat of weathershield paint first, wait for drying, dont need to paint 100% also nevermind. After 1 coat is done, if the weather is hot, paint a 2nd coat. Finish coats you need minimum 2 coats. Dont paint 1 coat, it will look ugly.

Sure it will last roughly about 5 years now, but also make sure you spray nicely la. XD Or else effort all gone. hahaha.. xD

Just saw your post and would like to share some information on this problem. This is efflorescent and is a very common problem in Malaysia, the root cause of this problem is actually moisture transferring the soluble salts from walls/plaster, seep out to the surface and crystalized to become the white solid. If there is no cracks on the wall surface for water to penetrate in, there are a few possible causes that may lead to this problem:

1. Improper painting system
2. Inferior sealer quality
3. High moisture level in the wall while painting
4. Direct contact to ground causing ground water to transfer to the wall

Now we know the root cause of the problem is the moisture inside the wall, assuming that the wall condition is still good, the best way to prevent it from happening is making sure the wall moisture content is always below 16% before painting. If the requirement in some circumstances are not able to achieve, base on my experience the solution would be by using strong alkaline resistance paint like this one, or any acrylic solvent-based wall sealer to replace the conventional water-based sealer. Alternatively you can also opt to apply a thick layer of acrylic based water proofing coating as sealer before subsequent painting if the wall moisture is constantly remain at the higher side.

Your solution should always be making sure the wall moisture level is low, and/or preventing the moisture from transferring the soluble salts out to the paint surface.

It is called protimeter and ranging from basic to professional. You can invest in a basic one to check if there is suspect water leakage or any types of wall damage due to moisture. In normal condition that you washed the wall/reskim, just leave it for one day to paint it on the next day.