A rock-like creature, Rockslide's background has not been revealed. His only in-panel appearance was a visit to Bruiser's, a well known social hangout for working class super-heroes.
His rocky textured skin would suggest extra strength and durability.
It's possible he made a second, equally brief cameo appearance as one of several characters being speculatively captured by the Conquistador. That would make him a borderline criminal, or a "black mask", not a hero. See Kurt Busiek's Astro City Vol.2 #19, page 16, far right bottom panel.