Not necessarily. That an app memory usage grows is not a sign of memory leak. It's normal for the app to start caching data and increase its RAM usage.Thanks. So it appears that there actually is a memory leak?
In the above screenshots, steam doubled its RAM usage from~600MB to ~1200MB, but that doesn't prove much (apart form the fact that apps tend to require too much RAM nowadays, but that's another story).
If one can find a set of operations that when repeatedly executed constantly increase the memory usage, then that could be a hint of a leak.