I don't know what it is called but I wish it wouldn't keep coming off the bolt/nut on my toilet. Is there a way to fix this? Get a better one? I don't know what to google for. Can you help?
A simple fix might be using white poster putty / sticky tack. If that doesn't hold, try a dab of superglue. Each of these are available at the dollar store. For a slightly more expensive option, perhaps some white adhesive silicone caulking. All of these options should hold the cap in place enough to not blow off in the wind, but stilll allow you to break open the cap again if you need to ever remove or tighten the bolts.
Decades ago the caps used to be made that fit tightly and never came off. The problem was, they were difficult to clean and people left cleaning solution around them, which would eventually seep inside and corrode the bolts to where they would break off. So the industry changed to leave the caps loose fitting so that when cleaning, it comes off and you can clean underneath and also make sure to get rid of any leftover cleaning solutions and let them dry before replacing the cap.