I recently decided to change out the group cap and when I removed the old cap I noticed a slimy whitish paste on the inside. Is this what scale is? I always assumed it was hard, calcium deposits more like cement.

calcium deposits = Scale

Is that calcium? It feels slimy and gooey. I figured scale to be like little stones. Hard to touch.

no.... maybe algae Clean it off and be happy =)

looks like scale to me. not enough to get excited about, but clean it off while you're in there.

That is some form of scale. I've noticed I always get a little bit up there when I pull my group cap (and I use softened water, though there is a modicum of minerals in my water—I use the Everpure Claris system that allows some minerals through at the setting I use). I believe that it's most likely that the scale formation you're seeing there is because of air in your group cap. Even though you bleed the group, I have little doubt that there are some pockets around the gasket and rim of the group cap, and that likely allows scale to form there even though it wouldn't form in the fully submerged portions of the boiler (i.e. the other 99.9% of your boiler, which looks clean in your picture). I would clean the scale if I were you, but I wouldn't worry about it. I don't believe it's likely to get much if any worse than what you've shown, provided your water is adequately soft.