Looks like to much water and not enough air circulation

I get that kind of thing sometimes when something strong or toxic has gotten into the cup, but other times it seems to appear for no reason that I can figure out, so I don't know what to tell you. I would rinse out out well, and as hotdiggety says, give it more air circulation, and hope it grows out of it.

Some of mine try to do that when its hot, humid, and rainy all at the same time. Only way I get around it is keep the air circulation up and the plants dumped daily and a good but light fungicide spraying. When its gets humid here the air stops moving, we loose our nice prevailing south winds

I would not fertilze this species, or at least only with a very dilute fertilizer.

Looks like alkaline water build-up, too salty. Those are the salts leaching out of the leaves.

Well,all of the above are possibilities for sure. I find Vriesea's to be touchy to having too strong a waterey fertilizer mix sitting in the cup for a period of time.Its ok for the root feed,but not in the upper younger leaves.
Lime build up from hard water may leave a residue,unsightly,but not harmful in my experiance.However,treated water,as with salt nuggets used in water softeners,can do some harm too,if left in the cup for too long a time.[a few days].Instead of using questionable water for those Expensive vrieseas,I catch rain water,or use melted snow in winter,or purchase distilled or spring water.In the long run,its less costly than losing a valued plant.Using oxalic acid treated water is new to me,so I have no idea what that may do.But it does'nt sound like a good gamble to use it on tender new leaf growth in plant cups.What I'm speaking of here,is mostly when plants are kept indoors,as in winter. When any leaf damaged plant is put outside for the summer,it recovers and grows back nicely,having the benefits of rain.But one thing new to us in the last few years,is something called Acid rain,where clouds pick up pollution from industrial factory smoke.That kind of rain will even destroy leaves on trees,if exposed too long to it.Mostly noticable on leaf tips,that drip off the excess water.
Not a perfect answer,but hopefully helpful. Bob

Thank you all for your answers. I crowded many of my bromeliads when leaving for holidays a month ago, so the air circulation was not goog for sure. Now what would be better, sun or shade? And should I get the cups to dry out?

Bob, I love to water my plants with rain water, but we have not had any significant rain since the end of April :-( Nobody seems to know the oxalic acid, I have only seen it recommended in one book, but while it is known that the household softeners will add noxious elements like sodium, or the use of sulphuric or phosphoric acid will add sulphur or phosphor in excess, the formula of the oxalic acid says there is only oxygen, hydrogen and carbonium, so it should be the perfect softener, but I am not a chemist.
Tomas

We use oxalic acid here to bleach wood, so I was surprised to know it was a water softener. Other reasons plants will rot are poor draining potting soil and flat surfaces instead of wire tables. This slows drainage and cuts down on air circulation. As far as water filtering I use the filters like used in a kitchen and change them out regularly instead of trusting chemical softners

Very limited quantity is needed - 1g for one galon, still the quantity of the lime that will deposit after one day is impressive. At least for my hard water. And it will lower the pH of 1°
Tomas