Does "target Merfolk" imply "target Merfolk creature"?
Yes, you can target either a Creature - Merfolk or a Tribal - Merfolk, and put +1/+1 counters on them. Rule 109.2 says


  If a spell or ability uses a description of an object that includes a card type or subtype, but doesn’t include the word “card,” “spell,” “source,” or “scheme,” it means a permanent of that card type or subtype on the battlefield.


And rule 308.2 says


  Tribal subtypes are always a single word and are listed after a long dash: “Tribal Enchantment — Merfolk.” The set of tribal subtypes is the same as the set of creature subtypes; these subtypes are called creature types. Tribals may have multiple subtypes. See rule 205.3m for the complete list of creature types.


So, any spell that refers to an object just by a creature subtype is talking about a permanent of that subtype. And since the Creature and Tribal types share the same set of subtypes, an object that is referred to just by creature type can be either a creature or a tribal permanent.