Catalina killing my 500GB SD storage

APFS has an unusual way of handling storage. That a portion is greyed out does not mean that is used. Every write to SSD takes a hit on its usable life, so APFS minimises the writes, but confuses expert and novice alike in the way it is presented to us.

If you truly need to resolve the anomaly you can force Spotlight to reindex. I did and my grey space became "vacant" again

The command that woodmeister50 has provided you should result with something like:

/dev/disk0 (internal):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme 500 GB disk0

1: EFI EFI NNN.N MB disk0s1

2: Apple_APFS Container disk1 500 GB disk0s2

/dev/disk1 (synthesized):

#: TYPE NAME SIZE IDENTIFIER

0: APFS Container Scheme - Physical Store disk0s2 500 GB disk1

1: APFS Volume Macintosh HD NNN.N GB disk1s1

2: APFS Volume Preboot NNN.N MB disk1s2

3: APFS Volume Recovery NNN.N MB disk1s3

4: APFS Volume VM N.N GB disk1s4

Does it?