What is the small glass-like crystal I see?

This can happen with naturally cooked tuna and is safe to eat.

These harmless, naturally occurring crystals are called struvite. They are formed by magnesium, ammonia, and phosphate which are naturally found in canned seafood. Sometimes during the cooking process, these elements combine to form what looks like small glass or clear plastic particles. Struvite is not harmful and will readily dissolve in the digestive juices of the stomach where the minerals can be absorbed by the body.