Hmm. First and foremost, are you preparing your feebas for evolution, and what is your native region? I ask because in some cases, feebas require a lot of effort to maintain. This is especially so in the Hoenn region, where you have two options for evolving your feebas. The first is by way of feeding it dry poké blocks to enhance its beauty. Granted, this method is less condoned because of the Pokéblock Maker Recall of 2007, but it’s still an option in rare cases. Even then, preparing for feebas’s evolution in general can be time consuming because the alternative method of applying a prism scale and finding a trading partner not only requires the obvious trading partner but also hunting down the rather rare and extremely expensive prism scale. Never mind, of course, the fact that it’s always a good idea to prepare your feebas for evolution by feeding it a specialized diet and maintaining its scales until they shine—which, incidentally, is also something you should be doing, even if you aren’t evolving your feebas.
In other words, feebas are a bit difficult to maintain, and it seems as if your grumpig is displaying classic signs of jealousy because he realizes this.
If this is indeed the case, then I must say that jealousy within a pokémon team can be tricky to resolve. While it’s prudent to give your grumpig a bit of extra love and care (to reassure him that he isn’t being replaced), it also won’t resolve things in the long term. What will may be some team-building exercises. Engage in double-battles with other trainers using your grumpig and feebas. Give them tasks that would require them to work together (as odd as that sounds, considering one of the involved parties is a fish). Have your feebas help your grumpig by offering to help him clean his pearls (using her water-based attacks, for example), and encourage your grumpig to do the same for your feebas. The main idea is have them spend time with one another and acclimate to each other until your grumpig sees your feebas as a teammate instead of a threat to his position on your team.
If, however, he’s not doing it out of malice, see if he’s curious. Ask him if he wishes to learn more about his new teammate and offer to teach him about her. Explain to him that she needs the water to live and show him her fins and gills to illustrate why. Then offer him more information about feebas and allow him to help you take care of her to educate him on feebas care.
Alternatively, observe his reaction at the exact moment you put your feebas back in the water. If he seems angry with you for doing it or if he looks even remotely worried that you’ve put your feebas back, test the water. Grumpig noses are more sensitive than a human’s, and he may be picking up on problems with your water quality, including pH imbalances or an excess of waste.