Wettest rainy season in my 5 years in Bali...
How to clean jamur (algae) off your palimanan stone steps, walls, statues, etc.:
1. Don't delay... the minute you buy your statue or build your temple or clad your balé in this lovely milky white sandstone, brush on silicone. You can get this at Cintra Warna on Jalan Imam Bonjol in Denpasar.
2. Okay, so you delayed and it rained. Your block of palimanan is now an ecosystem exploding like the Genisys Project in Star Trek. Don't even THINK of searching for Spock in all that green... just get it scraped off. If it's really bad, brush with dry kitchen/bathroom scrubber. Old toothbrush works great for those carved eyeballs and curley cues.
3. Resist the urge to hose it off. You will merely be feeding the algae. If you must use the water method, blast away with plenty of pressure and immediately follow up with step 4.
4. Mix 5 parts water with 1 part bleach and scrub, scrub, scrub. Don't let one drop of green remain. Leaving stains of black is not necessarily bad as this can add character to the stone, and you won't be able to get rid of those anyway, without actually scraping away parts of the stone itself.
5. Do rain stoppage dance until sun shines and dries palimanan.
6. Immediately coat with silicone and allow to dry.
7. Maintain stone steps by screaming at anyone who treads on them.
8. Maintain statuary by making daily offerings.
9. Move back to Australia or the States or wherever it is you came from, so that you'll never have to go through this nightmare again.