Weird black/grey dot on my photos?

That looks like a "dust bunny" to me.  I've had three Canon Rebels for over 4 years and only once did I find something like that.  It was on one side, so it wasn't critical that I remove it right away.  I was beginning to read about it, to find out if I could clean the sensor myself or have it done by a professional.  What I remember reading is to be careful to do it right, otherwise further problems could develop.  Fortunately over a period of weeks, the dust bunny got smaller and smaller and eventually went away.  That's one reason I feel the in-camera dust sensor protection is necessary.  I don't normally change lenses so that may also contribute to the fact that this rarely is a problem for me (I have multiple cameras with attached lenses).  But I read anyone can potentially get these dust particles (depending on their gear).  Once I had a dust bunny on my Kodak P&S camera.  But that went away on its own and then I stopped using the camera, because later new improved technology led me to other cameras.  That ended any future problems with dust with that.  My Olympus DSLR cameras had great internal dust sensor protection, so that wasn't a problem at any time.  I'm glad that my Canon DSLR cameras appear to be performing well that way also with this in-camera dust protection.

You could have the camera clean the sensor itself over & over and see if it goes away on its own.  Hopefully, you'll see it getting smaller and smaller and then soon it will be gone.  If that doesn't work, then cleaning the sensor yourself seems easy enough, but again it's important that you do it correctly.  There's enough information about this on the internet that should help you in that regard.  If you feel that you are not good at this type of thing, you might find a camera shop to do this for you professionally.  Check them out to see what price they would charge.  I think they would charge about one hour of labor or so.  It shouldn't be a lot of money, but I wanted to learn how to do it myself, because who knows how many more times it may happen.  I'm so busy, that I still have never done my own sensor cleaning.  Thank goodness for Canon's technology to reduce this occurrence.