Micro Seiki ma-505 x is a low or heavy tonarm?

That arm has a 14.5 effective mass. You can use this chart to calculate if the cartridge that you want to use would be compatible with that arm!

http://www.mh-audio.nl/Calculators/RF.html

Here ya go!

14.5 grams

Using a basic effective mass calculator (link below), the Micro Seiki MA-505 has an effective mass of 14.5 grams. It needs another ~16 grams to achieve a new effective mass of about 30 grams. To add 16 grams to this tonearm, more weight at the headshell and more counterweight has to be installed.

https://www.micro-seiki.org/denon-103-project

MA-505 Denon 103 Project — Micro

If you read this it states that the 505 has an effective mass of 14.5 grams. It might be that the 7 gram mass information that you have is not including the weight of the head shell!

MA-505 Denon 103 Project — Micro (micro-seiki.org)

 Here is a discussion about that arm. The head shell weighs 9.7 grams!    https://www.vinylengine.com/turntable_forum/viewtopic.php?t=48848