Ah, the first boss. There's a serious conveyance issue in it which is my fault - this will be fixed in the final version.
Damaging all four of these is necessary to defeat the Sub Teaser! Future releases will show you're making progress and dealing damage in the same way as is the case for the orbs on the far left and right, with screens that change to reflect they have taken sufficient damage.
My apologies for the confusion - entirely my mistake. I hope you've had fun otherwise!