As KJavalik pointed out in the Comments, it it safe, but Disk IO will increase and may choke other operations. So in case the Server is not serving any traffic/queries, you may try, else be careful.

Also keep an eye on the free disk space as optimize command will copy the remaining data to a new table, and if the slave is in replication, new data will also take up some space.

So in case you are optimizing 2 tables of 50Gb each, for example, and prior experience suggests that post optimization each table will be around 35 Gb, then its good to have at least 90Gb of free space (35 Gb + 35 Gb + 20 Gb of extra free space for other tables to grow in case the database is in replication)

As KJavalik pointed out in the Comments, it it safe, but Disk IO will increase and may choke other operations. So in case the Server is not serving any traffic/queries, you may try, else be careful.

Also keep an eye on the free disk space as optimize command will copy the remaining data of the table being optimized to a new table, and if the slave is in replication, new data will also take up some space.

So in case you are optimizing 2 tables of 50Gb each, for example, and prior experience suggests that post optimization each table will be around 35 Gb, then its good to have at least 90Gb of free space (35 Gb + 35 Gb + 20 Gb of extra free space for other tables to grow in case the database is in replication)