Here is one of the plants with the issue

Soil is of good quality (Happy Frog planting soil with composted material, humic acid, worm castings, guano, etc)

Classic nutrient deficiency. Problem is your soil can only feed them IF there is an active soil food web in the soil to digest them and turn them into nutrients the plants can use. That is how organics work.

But in a container, especially a small one like those, there is little to no soil food web to do the work. If you can get them into the ground ASAP the problem will resolve itself. Otherwise you need to use a liquid supplement - organic or synthetic - to feed them the needed nutrients.

Dave

Can you expand on the "active soil food web" ? And what the primary defficiencies are that would not be provided by a mix this seemingly well balanced? I have watered them once with a dilution of a product (Tomato Maker) which includes various nutrients and I thought that could be the issue because the whiteness appeared the next week. What liquid product do you use to supplement for plants in this stage of indoor growth?

If I have a soil medium with so much "good stuff" (I forgot to mention it also has Mycorrhizal Fungi), what is the missing component that makes it more "active" as you said? This would seem to be far better than topsoil or an average potting soil, or seed start mix and yet the plants look like they do.

I planted tomatoes year prior in a mix I made using box store composted manure/topsoil/peat mix with added Blood & Bone meal, small amount of lime etc and didn't have any issue similar to this.

To expand on the soil food web (aka soil micro-herd) here would take pages and pages. But it is a well known and well published concept with all kinds of info available online. It is discussed in expansive detail over on the organic gardening and the Soil and Compost forum for starters.

Over-simplified version - any dry-form organic supplement requires a well established active bunch of soil microorganisms, bacteria, fungi and 'bugs' to eat, digest and poop the organic matter converting it to a form of useable nutrients the plant roots can then absorb.

In the ground they usually exist but require several years of time and proper conditions to proliferate to an ideal level. In containers filled with any peat-based mix - which is what should be used in containers for many reasons especially drainage - they do NOT exist unless added (the small dose of Myco in this case). Even when added as in this case they cannot survive for long due to containment, the frequent watering, heat, etc. etc. The smaller the container the less likely they can function even after they colonize the root system and the shorter their lifespan. Your's have probably died off.

So one can generate an active soil food web in a 20 gallon container and keep it going but not in a 16 oz cup.

For that reason, the absence of the micro-herd, it makes no difference what the "good stuff" in your mix is if the plants can't access it. And for that reason IF organics is your choice then you have to use liquid suspensions and feed regularly. There are numerous recommended liquid nutrient organics available ranging from fish oils to seaweed and kelp blends, to worm teas, to compost teas, etc. - your choice. But they need to be liquid suspensions or capable of being dissolved in water so that the plant root can absorb them with the water. The standard recommendation is mix per label directions, dilute to 1/4 to 1/2 strength, and feed weekly.

The alternative which many organic gardeners use is to use synthetic liquids on the transplants while they remain in containers since those synthetics are more balanced and faster acting and then garden organically once planted in the ground.

Hope this helps clarify it all.

Dave