What is this engine part on a Ford Focus?

-I was driving my Ford Focus today and hit a pot hole. The car started making a noise. Turns out the noise was because some cap was rubbing against a belt. After the cap was taken off the belt, it sounded fine. The cap came off of something under the coolant reservoir. It has some type of fluid in it. It doesn't smell like oil or anything. What is this? What should I do?flat rubber cap with a groove around it grommet used to cover rust check hole dome type could be cap off strutt top therefor may have damaged strutDoes the cap have a Label on it. There are only so many things that it could be since you said it was under your coolant reservoir we will say that its not the cap to that. what about your windshield wiper fluid reservoir or maybe its your power steering fluid reservoir now that I think of it those are the only other 2 reservoir caps besides the coolant.
If you hit a pothole and the car started making noise, it wasn't any cap making the noise. You prob have front strut damage, it prob needs replacement.