HEC is a cellulose-derived thickener (cellulose being the plant fiber) that's widely used in cosmetics and paints. As a polymer for bubbles, it delivers a film that's very clean and stable, with less clumping tendency than guar gum.
Advantages and nuances
It hydrates more predictably and holds up better over time, which makes it convenient for formulas you want to reproduce with precision. In exchange, it tends to be a bit pricier and harder to find than guar.
Like all polymers, it shines in a blend. You can compare guar, PEO, and HEC in the KUANTIKA simulator to see which fits your goal and your water best.