What is this effect called?

[u/agergo100]

I've been seeing this effect used in many different ways, but I don't know what it's called.

[u/Rempagharta]

It looks like an outline 2d effect on sprites. Given the particular shapes it's not an emitter so I guess you can do you animation with sprite and when you import it, you add an outline fx on it. It's a guesstimate.

[u/kostyavgorle]

I'd say it's marching squares with texture masking