Is there a code on moshi monsters to get all the moshlings?

No.

There are seed combinations (sometimes called codes, seed codes, flower codes, flower combinations) for attracting moshlings. Some moshlings are given when a player completes a Super Moshi Mission. Some moshlings need a special flower seed which can only be received by obtaining a code in a certain way. Other moshlings require a code obtained in a specific way, possibly by making a purchase.