There are a couple key differences in the electrical characteristics of infrared LEDs versus visible light LEDs. Infrared LEDs have a lower forward voltage, and a higher rated current compared to visible LEDs. This is due to differences in the material properties of the junction. A typical drive current for an infrared LED can be as high as 50 milliamps, so dropping in a visible LED as a replacement for an infrared LED could be a problem with some circuit designs.
IR LEDs aren’t rated in millicandelas, since their output isn’t visible (and candelas measure light in a way weighted to the peak of the visible spectrum). They are usually rated in milliwatts, and conversions to candelas aren’t especially meaningful.