Urchin Question- Help!

[365seasons]

We recently purchased a Diadema setosum (Long-spinned urchin), and the guy at the store told us we could feed it strictly dried seaweed.
When we got home, I was researching them, and it says they survive off algae grazing.
Our tank is only a few weeks old, and therefore has accumulated no algae growth yet.
Will dried seaweed suffice, or is there something else we can do to help it?

Thanks so much.