The white stuff is albumin, a protein that sits dissolved in the fish's muscle water until heat forces it to unfold and clump into something visible. It isn't a sign the fillet went bad or was handled wrong. Every fillet does this. How much you see is mostly a function of how hard you cooked it.
Salmon just makes it obvious, because its flesh separates into wide, visible flakes instead of staying in one dense mass.
What albumin actually is
Albumin is a family of water-soluble proteins that coil into small, tidy globes at cold temperatures. Egg white is mostly albumin, which is why a raw egg goes from clear to solid white in a hot pan — same category of protein, same physical trick, different animal. Salmon carries it dissolved in the liquid inside its muscle cells, colorless and invisible while the fillet is raw.
Heat unfolds the protein and lets it clump together with its neighbors, a process called denaturation. A tangled clump of protein scatters light instead of letting it pass through, so it reads as opaque white rather than clear. That's the whole trick — nothing about it involves bacteria, spoilage, or the fish being anything other than fresh.
Why heat pushes it out through the seams
Salmon muscle isn't one continuous block. It's built from short segments called myomeres, stacked like shingles and separated by thin sheets of connective tissue. Heat makes the muscle fibers inside each segment tighten, and a tightening fiber squeezes out whatever liquid it's holding — water, and the albumin dissolved in it.
That liquid has nowhere to go but sideways. It travels through the gaps between segments, reaches the surface, meets more heat, and coagulates on contact. That's why the curds show up in a line along the seams between flakes rather than smeared evenly across the fillet.
Why more heat means more of it
The harder the muscle fibers contract, the more liquid gets forced out, and contraction force tracks closely with how far past done you push the fish. A fillet pulled off a gentle simmer or a low oven barely tightens before it's finished, so very little escapes. The same fillet seared hard, or left a few minutes too long in a hot pan, contracts fast and squeezes hard — the albumin gets shoved out faster than it can quietly coagulate, and it piles up in curds instead of staying put.
It looks like the fish is falling apart. It's really just protein doing exactly what protein does under heat, in front of you instead of hidden inside the flesh.