Why is there a solder mask opening over a microstrip line?
It appears that there is no soldermask on that board at all, and the opening you are referring is in the copper. When it comes to RF or other high-frequency signals, surrounding objects play a large role in the impedance of the line. If you have copper surrounding the microstrip trace(s), the impedance will be affected. Besides this, signals travelling on nearby copper can be coupled in to the high speed line, and this crosstalk could be disastrous for your high-speed signal integrity.
The reason the board doesn't have any soldermask at all is likely because of the reasons already mentioned - soldermask may change its properties over time, in different environments, etc. Eliminating it altogether ensures a more consistent impedance by doing away with that extra variable.