Yes he will attack sloops!

Does the Kraken only attack galleons?

Originally, the Kraken would only attack Galleons, did not drop loot and did not have the "slap attack." Krakens did not spawn in any of the pre-release versions, including the Final Beta, but were shown in teaser trailers for the full release.

What ships does the Kraken attack?

The kraken can spawn on any boat type, be it sloop, brigantine, or galleon - although the encounter is considerably less difficult while on a sloop - and you'll immediately know of its arrival when the water around your ship turns an inky black.