it is not for the calcium itself, but for the wrong ratio Calcium: Phosphorus (too much calcium compared with a too low phosphorus) and a high quantity of proteins. Clover is a legume, too, and for the same reason it is not recommended.
Guinea Lynx :: Hay Chart
if you see the chart you read that alfalfa has 16-20% of proteins (good when a body is growing up) and a ratio 5,5:1 which is excessive but can be good when the skeleton is growing fast. The good ratio for guinea pigs is 1.5:1 (about).
Clover has 16-18% proteins and 6:1 ratio Ca/P
Timothy has different and better values...
about the ratio what counts is the sum of the vegs,grass,hay, pellets eaten during the day. But usually grass, hay and good pellets have a good ratio. The problem is with certain vegs which are not balanced.
Pellets should be given in very limited quantity (or nothing at all), not more than a little spoon a day.
Alfalfa pellets can be useful if the piggies don't eat, is ill, is losing weight, but too many proteins are bad for the liver, the kidneys and cause obesity. I still have half bag of alfalfa pellets used in limited quantity after the adoption of my 2 months old and underweight piggie, but the vet suggested me to cut off the pellet letting her eat in a most natural way (huge amount of fresh grass and hay/vegs). And the pellets are still here... She is normalweight now and grew up amazingly developing a strong health (so far).
Now I am using as a treat a grainless pellet.
Let's say that grains, wheat and cereals are bad for all the piggies.
Legumes (clover, alfalafa, etc) are not good for the adults.