Hovering silverfish

I used TNT to clear a large area of space to create a lava pool and ended up blowing up monster eggs where silverfish were hiding. I tried to kill them, but they stood still, not doing anything, over lava. They didn't burn, take damage, or have sound effects, but I got pushed away when I tried to walk through them. It's like the mobs themselves froze while the rest of the game kept running normally.