In what order are the fears? Volcanic ash in the atmosphere, earthquake,tsunami, or damage to the local area.
Normally, damage to the local area with the birth of a new island just offshore or a major addition to El Hierro itself.
It's worth watching though since it could cause other volcanoes in the area to start up due to recurring earthquakes from this one (same thing occurring in Iceland). If that happens, who knows what the ultimate consequences might be.
I do know that we're seeing a major increase (> 50%) in earthquakes and active volcanoes worldwide compared to the past several decades.