It's likely either a misplaced tile put there by a map designer who did not realize the camera could scroll over there, or a legitimate glitch of the same kind that cause the so-called "secret worlds" in the first few Metroid games (generally some flavor of invalid or badly-mapped tile data that ends up resolving to the wrong or unusual tile graphics or collision data. Like this.
There are a few places in the game you can legitimately see this kind of bad/random tile data in game -- many of the larger rooms in the game have bomb-able ceilings that you can climb up into with the spider ball which lead to narrow, short, dead-end corridors. Sometimes you can see a row or two of random junk tiles above you.
An interesting (technical) read on Metroid II glitches can be found here, and this site has some interesting discussion of glitches that can be abused to enter these "secret worlds" (among other things).