https://en.wikipedia.org/wiki/Serum
"A buzzword for lotion in the cosmetics market"

I question whether "serum" is an English word that can refer to liquid cosmetics.
I suspect it's Japlish.
In OED, I don't think there is a sense of "serum" that is similar to 美容液:
.
serum
1a. Watery animal fluid, normal or morbid; spec. blood-serum, the greenish yellow liquid which separates from the clot when blood coagulates.
b. Therapeutics. The blood serum of an animal used as a therapeutic or diagnostic agent.
2a. attrib. (of 1), as serum albumin (cf. seralbumen n.), serum-globulin.
b. Used attrib. (with or without a following hyphen) to denote (the concentration of) substances in the serum.
.
In Japanese, the word セラム is used, and it often seems to mean much the same thing as 美容液.
But I don't think セラム means "serum" when used in the context of cosmetics.

gg5, eij

@@ -11,0 +11,1 @@
+<pos>&n;</pos>
@@ -12,0 +13,2 @@
+<gloss>liquid foundation</gloss>
+<gloss>beauty lotion</gloss>

http://ja.wikipedia.org/wiki/美容液