Jimmy discovered this scenario long ago — the Heart Eye Vision isn't all-powerful. Once items are connected, the outline displayed can't be split in the middle and thus can't be recognized.
For instance, a suitcase with uniform sides stuffed inside placed on top of a wardrobe would appear to Jimmy as a single piece, just a wardrobe with a full shelf, unlike when viewed with direct vision where the upper and lower parts can be distinguished.
These Spider Monsters give Jimmy a very bad feeling because once outsiders capture them, it'll cause big problems. Cleaning them up as much as possible gives him some peace of mind. Even if he discovers afterward that he hasn't cleaned thoroughly, it's far better than doing nothing at all.
