I generally don't have an issue with someone borrowing my lessons but I think courtesy would dictate that the person ask, particularly if they decide to use the visuals you have put together. However if they just take the idea and work on their own visuals it's hard to say much.
On the topic of doing the lesson with another grade level, I have a teacher I work with who does the same thing. She will use a lesson for more than one grade. Which means the following year you can't use that lesson again because two grades have already done it and don't want to do it again. In the long run it means more work because you have to come up with more new lessons the following year. I always come up with new ones anyway but it's nice to have the old standbys if need be. I have pointed this out to her and she agrees but still uses the same lessons across 2 or 3 grades.
Our assistant superintendant wants us to put lesson plans in with the curriculum we are now developing. His reasoning is such that when he hires someone they then have something to start with. My comment was "If you are hiring someone that needs these lesson plans to get started in this job you have hired the wrong person".
I came to the job with knowledge and lessons to get me going I would think any good art teacher would do the same.