It seems that “Rheum” is the technical word to express
a thin mucus naturally discharged from the eyes, nose, or mouth, often
during sleep (cf. mucopurulent discharge)
Dried rheum is commonly called sleep, sleepy-seeds, sleepy
buds, sleepy sand, sleepies, eye boogers, eye crust, eye goop,
sleep dust,  or sleepy dirt.
Among those words, which one is the one we often say in everyday conversation?
You can choose any of those. If you want someone to pick one for you, pick the first one: “sleep”.
He yawned and rubbed the sleep out his eyes. Blinking, he looked around the room. Yes, the penguin was still there….