I wouldn't be too concerned. There's at least one missing on half of the cars I see in the shop. If you are worried, use compressed air to blow any debris or moisture out of the hollow part of the bleed screw, and put the cap back on. (A can of keyboard/computer cleaner spray has the perfect-sized tube for this purpose if you do not have a compressor and air tool.)

Do NOT crack the system open to "clean" the bleed screw unless you have a pressure bleeder, and are 100% confident that you can bleed the system correctly and safely.

Most people don't even know what a bleeder screw is! If you are losing sleep over this it would have been easier on you to take 15 seconds and turn the tire to get to the caliper and snap the cap on!

By leaving the bleeder open to the elements for so long, you have exposed it to dirt and corrosion that may make it more difficult to bleed the brakes next time. I'd open the bleeder for a few seconds to flush out any dirt and crap inside before you seal it up with a new cap. (Make sure there's pressure on the line before you open it)

Very few cars even have bleeder caps. Put yours on and forget about it.

I changed the front brake pads in my '08 Corolla a few months ago. Doing so I broke the bleeder cap. This is the first chance I've had to get in there and replace it, even though I've had the part for months. Is there something I need to clean or should I just stick it on there? Is taking so long to replace it a serious issue?

*I know, I should have done it sooner. What's done is done, so just straight out answers, and no bashing please.