Welcome to Reef2Reef!
Nothing a few water changes and trace element dosing couldn't fix :)
I assume you are using something like All for Reef, Balling, RedSea foundation, ATI Essentials, etc. to maintain alkalinity and calcium? It appears you add too much and need to reduce it for now. This would explain the high calcium and alkalinity levels while salinity is low.
For measuring salinity there are multiple options, but in my experience glass hydrometers cause the least confusion after people learn how to use them properly. Refractometers and conductivity meters need to be calibrated, and the automatic temperature compensation doesn't always work as intended. A glass hydrometer will give repeatable results for years, which is more important than 100% accurate results for this hobby.
This thread is going to be spammed with welcome messages and you might want to open a separate thread if you have further questions.