I am not sure this is a Melee attack. Bullet for Surt is stopping other attacks when I fight a Writhing Mass, but not this one. Maybe it's untyped?
You fire a heavy round into the writhing mass for 16 damage.
The writhing mass grabs you with a tentacle and pulls you closer to its snapping maw.
I think you can avoid it if you oppose it with Melee and have enough Strength or use Evasion. My Strength right now is 17, but I'm not fighting the tentacles off when it grabs me, which I suspect has something to do with the fact that I'm using Ranged attacks.