The algren came at Gramps and his family.
When one got close enough to touch Cathy's shoulder, Gramps, still invisible, blasted it to smithereens.
He wanted them to think Abby and her family were surrounded by some disintegrating force rather than just Gramps firing off a weapon. He figured the lame ruse would allow them at least a few extra seconds of life.
The algren retreated a step or two.
One thing for sure: he'd take down as many algren as he could before they ripped him apart or blasted him to pieces. No one was touching a hair on any member of his family if he could help it.