Naruto 465 Spoiler

溶遁 youton
mizukage's may be from the word youkai
溶解 - youkai - melt/liquefy
probably water + fire chakra natures

熔遁 youton
roushi's is from the word yougan
熔岩 - yougan - lava/molten rock
confirmed as fire + earth chakra natures

both jutsu would have similar results, so i guess he thought they're names should be similar too.

to avoid confusion, i'm gonna refer to them as molten release (mizukage's) and lava release (roushi's).

ShounenSuki explained the difference between 溶 and 熔 already.