Ideal condition to make injera

The way I make injera is simple. Make a batter teff and water. Once your batter or dough is done take half a liter of water and pour it over your batter or dough. Let it seat for 5 days. After 5 days, REMOVE the separated water and keep it aside. You will need it to cover the batter again once you done cooking. Mix well the batter without the excess water and cook. Add new teff flour to replace what you took to cook.

THe batter ferments UNDER the excess water.

I haven't done injera yet but I make dosa which is similarly traditionally fermented and one of the most important factors to do this safely and effectively is temperature. My experience with dosa batter indicates that much lower than ~70F (~21C) really slows the fermentation down. I tried to find a more precise temperature recommendation and found a blog (Culinaria Eugenius) which recommends 30C (~86F) (and includes a really thorough walk-through, too).

I have a really good Ethiopian cookbook at home I can check later, but I don't know if it will have a precise temperature recommendation.

If your kitchen is on the cool side (and I hope it isn't a steady 30C for your sake :-) ), it may be helpful to ferment it in the oven. If you have a pilot light on a gas oven, that is enough heat to keep it warm enough, or if not, if you have an internal oven light you can turn on, that can be enough heat.

You would need to keep it out, covered, for some time first, though, as the oven is less hospitable to the wild yeasts you want to catch in the first place.

Your batter could have been ready after 2 days, although it will achieve a stronger flavor if you let it ferment longer.

Another recipe from Mesob Restaurant in NJ says that it can be anywhere from 2-6 days for it to complete fermentation, but for safety's sake, you really do want the fermentation to start quickly so that the good wild yeast can build up and get finished before anything bad gets a chance to take hold, especially in a humid environment that is friendly to such mold spores.

(Note, Mesob is a good restaurant and their injera is good, even if they do cut costs by mixing in other grains, so I trust their advice even though I haven't made it myself from this recipe.)

Regarding your current batch, another blog instruction for making starter says that you can simply remove the mold by skimming off the top. If there is enough alcohol in the mixture that may be safe, but I personally would be likely to throw it out and start over, which is what is generally recommended in answers to this question about sourdough starter.

Edit: None of my books say anything more useful than "room temperature" for fermenting. Most indicate that it should be ready in 2-3 days.

I have speculated about the reason for fermenting injera with sufficient water (water:teff weight ratio = 2) to form a layer on top of the batter.

After watching the batter slowly ferment for a few days I noticed that on close inspection what I had previously thought was a few patches of bubbles were not bubbles at all but perhaps a surface mold that was feeding on the nutrients brought to the surface by bubbles from the fermenting batter below.

I found that I could remove most of the surface mold with the edge of a folded paper towel, then after wiping the sides of the fermentation container as clean as I could, I was able to pour off the remainder of the liquid and have what appears to be a perfectly good batch of injera batter.

I suspect that the mold is a contaminant in the teff flour and something that you can't really get rid of. So the liquid cap layer is a feature rather than a defect in the design. It also turns out that a lot of the bacteria reside in the liquid layer so you want to carry some of it over in your ersho (starter) to the next batch.

There is an outstanding publication on Ethiopian fermented food containing a fairly complete section on injera, see: Ethiop. J. Biol. Sci., 5(2): 189-2245, 2006 [the section on injera begins on page 205] and you can find more of my notes at https://www.thefreshloaf.com/node/69026/injera

I too had this issue and also began with a teff starter (ersho). I easily resolved the mold problem by pouring a thin layer of live whey / live kombucha on top. What this does is that any sugar water that separates from the teff will easily be digested by the lactobacillus or SCOBY, preventing the molds from consuming the sugars and growing. The acids produced by LAB/SCOBY also deter unwanted, foreign competition.

Injera is naturally sour anyhow so the flavors blend quite nicely. If the additional sour is not wanted, simple ladle it off when you wish to cook the injera. Still though, the probiotics and acids are healthy - take it as an exercise in furthering your appreciation for sours :)