The episclera covers the sclera (see Eye – Anatomy). If there’s a patch of dull redness on top of the white sclera, it looks like it’s part of the conjunctiva but is actually underneath. There are usually tufts of tiny blood vessels branching out. The cause is unknown in most cases, though it may rarely be associated with certain types of inflammatory arthritis.
The condition is completely harmless, usually has no real symptoms, & eventually goes away on its own. Eye specialists can diagnose it easily with their special equipment. Regular clinicians aware of the entity will identify it with just a simple penlight, & by knowing all the other conditions that it isn’t. Here’s a picture showing how the redness is localized in a patch, with networks of blood vessels if you look closely enough (as opposed to a Subconjunctival Hemorrhage, which is socked-in red; or an Inflamed Pterygium, which is in its own special location).