What happens inside food when heat, water, mixing, fermentation and other cooking processes change its structure.
When rice that should be fluffy turns mushy, the usual problem is too much moisture plus too much gelatinized starch around the grains. Excess water, overcooking, repeated stirring or a naturally sticky variety are the main causes. Rinsing can remove surface starch, but it cannot fix an overly wet ratio or make sushi rice behave like basmati.
The white material that appears on salmon during cooking is protein from the fish pushed out with water and then coagulated by heat. It is commonly called albumin. It is safe to eat and does not by itself mean the fish is spoiled or contains parasites. More tends to appear when cooking is especially hot or prolonged.
Rubbery chicken does not always mean the same thing. Dry and stringy usually points to overcooking; a springy or translucent center may need more cooking; a breast that felt oddly hard before cooking can point to woody breast.
When a skillet turns into a puddle, it is tempting to blame the meat. But muscle is already about 70–75% water; the real question is how much moisture comes out and whether the pan can evaporate it fast enough.