Molding ASCII art

LabVIEW, 37 LabVIEW Primitives

splits the string into text and mold then transforms them into an array. Checks the mold if theres a # and puts a char from text else it does nothing. If either the text or the mold are empty exit the loop

Haskell, 48 bytes

called like "(replace with string)#(hashmark string)":

[]#_=[]
(r:t)#('#':s)=r:t#s
r#(c:s)=c:r#s
_#_=[]

Less golfed:

combine replacements slots | null replacements = []
                           | null slots        = []
                           | head slots == '#' = head replacements : combine (tail replacements) (tail slots)
                           | otherwise         = head slots        : combine       replacements  (tail slots)

CJam, 16 14 bytes

Thanks to Sp3000 for saving 2 bytes.

lq{s'#-\+(\}/;

Terminates with an error if the string is too short, but the error is printed to STDERR.

Try it online!

Alternatively (same byte count):

lq{SN+&\+(\}/;

Explanation

l       e# Read the input line.
q       e# Read the grid.
{       e# For each character in the grid...
  s     e#   Convert to string.
  '#-   e#   Remove "#" from it.
  \+    e#   Prepend it to the input line (this is a no-op for "#"s in the grid).
  (     e#   Pull off the first character from the input line. This will terminate the
        e#   program with an error once the input line is empty.
  \     e#   Swap the character with the input line.
}/
;       e# Discard the remainder of the input line if there is any.