The issue I see from your error is that, you have given a wrong datafile name (since its in ASM, it doesn't require a '/' in the beginning and the ".dbf" extension in the end) and the syntax needs a little change.
You must be giving it as below:
SQL> ALTER DATABASE DATAFILE '+PROD_DG1/prod/datafile/d_cqm.621.936232125' AUTOEXTEND ON NEXT 50M MAXSIZE UNLIMITED;
"NEXT 50M" - tells the database that the segments should be allocated in 50M chunks as the datafile grows -> if not specified it will increment by 1 by default
"MAXSIZE UNLIMITED" - tells the database that the maximum size of the datafile can be up to the allowed size based on the formula (4194303 * db_block_size parameter). --> this is a default value even if you don't specify MAX_SIZE
if db_block_size parameter is set to 8kb - then your datafile size can be up to 31.99 GB => 4194303 * 8192 = 34359730176 bytes ~ 31.99 GB
And you can refer the links suggested by Burleson for more accurate understanding.PS:
I am a DBA from CERNER