SQL server by DESIGN will always eat up as much ram as you throw at it if you have databases of sufficient size. Have a 32GB database and 32GB of memory ... SQL by default will eat all of it.
You can configure SQL to limit the amount of memory it will consume however. Lets say you have 16GB of memory in your system try limiting the SQL buffer to 10GB, that will leave 6GB or so for the host OS, other SQL processes, and any other programs to use ( ex: your other SQL express instance). Then monitor the memory consumption on the system and then tweak your buffer up or down depending on how much memory you still have available.
So you dont need to worry about it.SQL Server is usually pretty good at sharing RAM and managing memory in general.SQL Server is a good neighbor, though, and if other programs want some memory, SQL Server will free up some memory for them—most of the time.
Real issue with SQL server arise when CPU usage goes up