You are right to quarantine the plant from others.
The magnified photo showing the underside of the leaf is out of focus, but it appears there might be an infestation of mealy bug.
The photo showing the top of the leaf indicates damage throughout the leaf structure that I though appears consistent with spider mite infestation, however this can also be caused by mealy bug.
Please attempt to upload a photo of sharper focus to the underside of the leaf for confirmation.
If this is mealy bug, then I'd recommend...
Chemical treatment: a half dose (due to age of plant) white oil spray over every part of the plant.
Organic / biodynamic treatment: a half dose (due to age of plant) neem oil spray over every part of the plant.
Adding a very small amount of detergent will assist coverage of leaves and pests as it destroys the natural surface tension in water. This also helps to suffocate pests - instead of forming droplets/bubbles on objects, water collapses and literally drowns pests.
Don't use half strength fertiliser. Tomatoes, even as seedlings, are hungry plants and need a lot of food.
Also consider applying a "tonic" that might contain seaweed and/or soil micro-organisms in suspension.
I'd highly recommend planting basil amongst your tomatoes. The aromatic oils emitted from the basil help to repel potential pests and make the tomatoes taste so much sweeter.
"Tonics" that contain seaweed or seaweed extract are:
- a good fertiliser that will support strong and robust plant growth; and
- a good soil builder to help improve the organic matter content of soil and soil microbiology.
Seaweed and its extracts have a broad and balanced range of nutrients and importantly trace elements.
Peter Bennett's book "Organic Gardening" taught me how trace elements facilitate or catalyse the "uptake" / absorption of both nutrients and other essential trace elements.
For example, certain ratios of trace elements that benefit absorption of ionic Magnesium can benefits plants, as Magnesium is essential for many plant functions, including:
- Photosynthesis: Magnesium is the central element of the chlorophyll molecule;
- Carrier of Phosphorus in the plant;
- Magnesium is both an enzyme activator and a constituent of many enzymes;
- Sugar synthesis;
- Starch translocation;
- Plant oil and fat formation;
- Nutrient uptake control;
- Increase Iron utilization;
and the list goes on...
So healthy soil containing a broad and balanced range of nutrients and trace elements will help your plants grow strong, fruit/flower well and be resilient to pests and disease.