Glueing a mushroom is like trying to glue a bugger to silicone.. One way is to place shroom in an area with little to no flow so it does not move or float around and let it be for a few weeks or so. In this timeframe it will start adhering to the sand its setting on. Now you can pull it out, pat the bottom slightly with a paper towel apply superglue jel to thr attached sand and stick it wher eyou want. Chances are if you simply apply glue to the shrooms base without any sand adhered to it, its gonna move of...
Another quick and effective way to put it in place and have it stay there is to use a hypodermic needle. A long perhaps 1 or 1 1/2" of 22 to 25 gauge needle is whats needed. Also a piece of a rubber band, that is perhaps 1/8" wide and about the same in length, just a small tiny pice that you stick the needle through, and slide it up the needles shank. Then stick the shroom near the center but not through the mouth portion. NOw place shroom where you want it, and push in on the needle so it goes into the liverock. It may sound like its not gonna stick in live rock but live rock is fairly solf and you should be able to push the needle in sufficiently for it to take ahold/ Push shroom down so it contacts rock surface, and then push rubber band piece down till it just touches the shroom. The rubber band piece is a stopper so it keeps the shroom from floating up on the needle. In about a week or two the shroom will have adhered to the rock and the needle is easily removed.........I used to make little net cages over ricordea in a bottle cap to keep it in place until it attached to sand, then remove net cage and glue where I wanted....... now I use the needle method, and my ricordea is where I want it and no glue needed........most adhere in a week or so just fine. Its simple and works fine. If a hypodermic needle is not available without jumping through a lot of hoops a sewing needle can be used if you put a piece of plastic or other material over the eye end so the fish do not get stuck or jabbed with it.......and look for polished stainless steel sewing needles, but hypodermic needles are available at most any pet or vetrinarian or farmers supply