Yes, but it would be such a limiting form of communication nearly any alternative they could improvise – banging sticks on the ground or against their shells, for example – would be more robust.
The downsides of slime
Body language would be necessary to signal that one is about to 'transmit' and the intended receiver should be prepared to 'receive'. An intended recipient who isn't present or isn't looking the right direction would be oblivious to this.
You can't communicate to more than one receiver at a time, and they must be immediately present and willing to receive. These snails can't shout. An intended recipient who doesn't want to receive the message can simply refuse to track the slime message, and the sender can't override this without physical violence.
Receiving a message is labourious: depending on information density you may have to follow in the slime track of the sender, unable to scan ahead.
Assuming a linear method of encoding, there can be no interjection, no proper dialogue: A sends in full, B receives, B replies in full, A receives. Literally snail mail.
The upsides of slime
- The message can endure until the slime degrades or is mechanically disrupted enough to obscure the contents. Bad for privacy, but good for leaving messages to a receiver who isn't yet present.
Improving on the familiar gastropod
Rather than leaving trails in the dirt as our snails and slugs do, your xenoslugs have evolved their organs to fire info-dense packets of slime at each other, with great accuracy over several metres. Much of their exposed tissue is porous and receptive to this, skin composed of tastebud "ears".
As a medium, the complexities of the language could be well outside our capacity to understand. Think polysynthetic, but in multiple dimensions: scent, colour, viscosity, density, chemical reactions, etc. The tiniest squirt could take thousands of words to translate.
This still wouldn't be as good as vocalization in many respects, but orders of magnitude better than slime trails.