I am also trying out my first batch of sake and am wondering what the kome koji is soposed to look like when it is ready to use.
See these two links
https://www.nrib.go.jp/English/sake/ss/sshm_kome-koji.htm
and
https://homebrewsake.com/recipe/video-series-and-instructions-for-making-koji-for-sake/
My own tips in short words are....
1. Only use SUSHI rice and never used any rice stating 'sweet/sticky/glutinous' for the Koji making. You want rice that does not stick together and will separate easily. If you see videos on pro brewers raking koji rice around it will look like dry rice and will never stick or clump. So use sushi rice.
2. Wash rice till it clears, soak it the correct amount and then steam for the right amount of time. I have found the normal tap water temp works for me at a 1.5 hour wash and soak combined, then drain for an hour and steam for 1 hour. Soak the rice until it has no crunch left in it, but it will be crumbly when you bite it. Then no water should come out when you shake the rice in a strainer. After steaming the rice should be like rubber and not soft and fluffy like you normally eat rice.
3. You really need to keep the rice at 30 degrees Celcius when growing the koji, only at a certain temp range will it produce the Amalayse that breaks the rice down. You can not do it at room temp. I use a polystrene box or beer cooler and place hot water bottles in it with 50 degree C water and it works great. Change water every 12 hours.
4. If the koji goes green/yellow it is going to spore and should not be used for sake making as it can be bad for you. White or Tan is fine, just no black, green or bright yellow. I am growing some of mine on to spore stage at the moment to see what happens and I may dry and save it for making another batch.
5. I have been growing mine for 3 to 4 days and use it when it starts to taste sweet, I find at day 2 it has the mould and a slight smell, but by day 4 it is very strong smell and you can not mistake the sweet.
6. Make extra and freeze it, if you need to sweeten the sake after it finishes fermenting, adding more koji makes sure it really it finished and it did not just run out of koji, the extra then sweetens the sake and creates a better mouth feel, gets rid of the watery taste and balances the sour taste. Making extra to fake dry age steaks or to play with making spores, once you get the koji growing well it is easy to make the sake as it is just repeating the steaming of the rice.