Looks like mildew. Is the floor tile or vinyl? Bleach should get if off.

If it won't come off then most likely water seeped underneath the linoleum and has molded.

"If it won't come off then most likely water seeped underneath the linoleum and has molded."

Huh?

No wax vinyl flooring can have a wear layer on the top and I think water can seep under the edge of it. I once knew a woman who cleaned and machine buffed her no wax floor so much that she took the wear layer right off. It was peeling.

How is mold beneath linoleum visible from the top? If it is on top it is on top, linoleum is not see thru.

I’ve seen a clear wear layer on vinyl.

https://www.bestlaminate.com/blog/wear-layer-vinyl-flooring/

How vinyl is constructed.

To understand what to look for with vinyl flooring, it is useful to first understand how it is constructed. Most modern vinyl is technically “composite” vinyl, meaning it is a combination of PVC (polyvinyl chloride, the scientific name for vinyl), plasticizers, and print films. The product typically comes with a backing material, which is a mix of vinyl and plasticiser or fiberglass. That material is then covered with a paper print-film, which has the desired color pattern. On top of the print-film, a clear layer of virgin or recycled PVC (vinyl) is layered as the main protection to the floor. This top layer of pure vinyl is called the “wear-layer”, and is the most expensive part of the flooring, since it is the only thing protecting the paper print film. Typically the wear-layer is given some level of texture or “embossing”, to better compliment the look of the print film being used. Finally, a coating of urethane is applied to the wear-layer, to further protect wear and provide the desired sheen for the pattern. The whole product is then cut up into smaller planks or tiles, depending on the pattern.

It’s not vinyl, it’s linoleum. So we think it’s mold?

Yes, it looks like the mold that grew on my Marmoleum when my dog spilled his dog water on the little rug it sat on. We did not realize that it was wet, and it stayed wet for weeks or months. He was old and sicker than we knew and may have been peeing on the rug, too. (It was so watery from all the drinking that it did not smell.)

For others who are going on and on about vinyl, this is not vinyl. It is linoleum, a natural product made of cork and wood dust, colorant, glue, and jute backing. There are no petroleum products or plastics and it is not waterproof.

So will bleach remove it? Oxyclean?

"millworkman

How is mold beneath linoleum visible from the top? If it is on top it is on top, linoleum is not see thru."

While linoleum is naturally resistant to microbes, it can grow and harbor mold in the right conditions. Linoleum is made up of limestone dust, pigments and linseed oil on a jute backing. If moisture and mold have gotten beneath the linoleum, the mold may grow right through the tile to its surface.

https://homeguides.sfgate.com/tell-mold-under-linoleum-71557.html

Just throwing out another suggestion. Not sure if it is a possibility or not. I’m thinking you’re getting condensation running g down the headboard and behind the baseboard it’s soaking into the back of the baseboard and what you’re seeing is the staining from dissolved glues and resins from the composite baseboard seeping our from the corner.

Beadboard. Darn spellcheck!

You might want to carefully pry off the baseboard and find out what’s happening at the bottom corner of the wall there. If there are no issues, it’s easy enough to put the baseboard back, caulk it properly and give it another coat of paint.

A suggestion on trying to remove that stain is to try a little auto wax and the Mr Miyagi method of wax on, wax off. I thought maybe some Spot and Tar Remover but that may affect the flooring.

If nothing else, you’ll soon find out if it’s staining on top of the Lino or within it. That’ll dictate your next course of action.