If there's more water in the atmosphere due to global warming, how are there fewer mountain glaciers?
Indeed some glaciers are growing and gaining mass due to increased precipitation (in part due to climate change and enhanced atmospheric water content). But that's the case only for glaciers in very cold places, like East Antarctica, where most glaciers appear to be growing at increasing rates (1,2). But unfortunately West Antarctica is pretty much a desert; the total precipitation on average is less than in the Sahara Desert. That is usually the case in very cold places. The capacity of air to hold dissolved water decreases exponentially at lower temperatures. Therefore cold glaciers (the ones that are most often growing) account for a small amount of the total global ice mass change.

In contrast, temperate glaciers (i.e. not so cold) constitute the majority of the most well known mountain glaciers (outside Antarctica and the High Arctic). These in many cases are receiving more precipitation, but the increases in accumulation are in most cases offset by the increase in temperature, which have a double impact:


More melting due to higher temperatures
A larger fraction of the precipitation falls as rain instead of snow, therefore promoting melting instead of accumulation.


That last point is not that intuitive, but as explained in this question and answer, warmer air carrying more moisture will develop water droplets quicker, therefore dropping most of the extra moisture at lower elevations. As a consequence, the rain-driven melt in the lower areas of the glacier will overcome the a moderate increase in the accumulation at higher elevations.

Also, there is a positive feedback kicking in against the glaciers: The more the glacier melts, the lower its surface will be, therefore it will be exposed to warmer temperatures leading to further melting, leading to more surface lowering, exposing it to even higher temperatures, producing even more melting and so on.

It is also important to note that glaciers grow due to snow accumulation. It is unclear to me if that's what you meant by "moisture condenses at mountain tops".

Finally, it worth noting that glaciers have been receding since the last ice age about 20,000 years ago (as we have learned from Oxygen isotopes on foraminifera deposits and glacier deposits), the recession slowed down the last ~6,000 years slowly approaching to a steady state. However, recent warming has just accelerated the process a bit, enhancing the extent and speed of the natural deglaciation, but we have to acknowledge that the glaciers were already out of balance and shrinking way before the industrial revolution.