Think of subconjunctival hemorrhage as a bruise on the eye. Because the blood is overlying the white part of the eye it's appearance is very dramatic. And like a bruise, it is not a serious condition but rarely may be indicative of a bleeding disorder.
A hyphema is a more serious injury to the eye but fortunately much less common. Unlike a subconjunctival hemorrhage, a hyphema is bleeding behind the cornea and is often associated with direct trauma. My understanding is that a hyphema is possible with mask squeeze but the squeeze must be severe, as in an unconscious diver sinking without any equalization.