Try this:
zm --server ssl://eu.zec.slushpool.com --port 4443 --user userName.workerName --pass workerpass
# zm 0.5.8
# GPU0 + GeForce GTX 1070 MB: 8192 PCI: 1:0
# telemetry server started
# encryption enabled
# connected to: eu.zec.slushpool.com:4443
# server set difficulty to: 001dca01dca01dca01dca01d...
it works for me
This worked for me. My ping is around 300, but I'm in South Korea. The other way didn't work at all.
Nvidia cards (four 1070 OC)
Linux/Ubuntu 16.04/DSTM 0.5.8
Very important with slushpool, to have the period between the username and workername. I has a '/' before like nanopool and it will not take. My pass coda was 'z', not "workerpass"